Explanations for the 2013-2014 Official USMLE Step 1 Practice Questions

From the official 2013-14 “Step 1 Content Description and General Information” booklet (archived here), I’ve listed the correct answer for each question with a brief explanation ± take home points. I can’t reproduce the questions themselves of course as they’re copyrighted.

The newest set of official questions (2014-2015) has been released. My explanations can be found here.

For those who have done the more recent set, a list of the questions unique to the 2013-14 practice materials are found in this footnote.1

Block 1

  1. D – Volume loss (secondary to vomiting in this case) causes hypovolemic hyponatremia. The body wants to retain fluid and maintain blood pressure, so there is a compensatory increase in anti-diuretic hormone (ADH/vasopressin).
  2. E – Xanthogranulomatous pyelonephritis (XGP) is a rare form of chronic pyelonephritis, especially associated with Proteus infection. Tumor-like growth, upper urinary tract infection, and (this is key) lipid-laden foamy macrophages make this neither acute pyelo nor cancer. Malacoplakia causes GU papules/ulcers, typically of the bladder (not super important).
  3. E – Wernicke–Korsakoff syndrome causes degeneration of the mammillary bodies (important fact). Korsakoff’s Syndrome on the boards invariably includes anterograde amnesia with confabulation (making things up).
  4. E – A chronic “heaped up” ulcerative lesion of the skin, especially on sun-exposed areas in people who spend time outdoors, means cancer. If it’s keratinocytes at play (and not melanocytes), that leaves you with either squamous cell or basal cell carcinoma.
  5. A – Gram-positive cocci in pairs and chains means streptococcus. Group B streptococcus can be part of the flora of the mother’s vagina (which babies are exposed to during vaginal birth). It’s the most important cause of neonatal sepsis and meningitis.
  6. A – Naughty alcohols–methanol (wood alcohol), ethylene glycol (antifreeze), and isopropyl alcohol (rubbing alcohol)–cause terrible damage through their metabolites (formic acid, oxalic acid, and acetone, respectively). The trick in treatment is to prevent the conversion of the original alcohols into their toxic metabolites. This can be done by saturating the alcohol dehydrogenase enzyme with intravenous alcohol or through the use of fomepizole (a competitive inhibitor).
  7. D – HPV (particularly strains 16 and 18) causes squamous cell carcinoma of the cervix. Thankfully, there’s a vaccine for that now.
  8. B – You don’t necessarily have to know a ton clinically about differentiating endemic fungal diseases, but you do have to know which areas they come from and what they look like. Coccidiomycosis comes from the southwest (i.e. California). Histoplasmosis is endemic to Ohio River valley. Blastomycosis on the east coast. Paracoccidioidomycosis from Mexico and South America.
  9. B – Esophageal varices are a big deal and can be fatal. The left gastric vein is to blame. Periumbilical varices can cause caput medusa. Superior rectal varices cause internal hemorrhoids.
  10. D – Antibiotic-associated diarrhea caused by clostridium difficile can be tenacious, difficult to treat, and even fatal. Alcohol-based rubs are not sufficient to kill the spores. Handwashing with soap is necessary, and equipment should be autoclaved to clean it.
  11. C – Lymphatic spread of disease moves through lymphatic channels from distal to proximal. Just follow logic. In the lower extremity, that’s foot to knee to groin.
  12. C – Thankfully, painful biochemistry really isn’t all that common on Step 1. MCAD is a fatty acid oxidation disorder. These disorders can be fatal, as affected individuals cannot break down fats into usable energy during times of fasting/starvation. None of the other choices are fatty acid oxidation disorders (and consequently associated with a build-up of fatty acids)
  13. E – Zoster is caused by a reactivation of latent varicella virus (aka chickenpox) from dorsal root ganglion.
  14. C – Blood flow also increases during exercise. The more anaerobic metabolism you use and lactate build-up you have, the more hyperemia you need to clear out the waste products.
  15. D – Irritable bowel syndrome is a diagnosis of exclusion (you have to make sure they don’t actually have an organic bowel pathology, like IBD). It is characterized by alternating bouts of diarrhea and constipation associated with psychological stress.
  16. A – The correlation coefficient, R, describes the degree of association of two variables. When positive, the association is direct (positive). The closer to 1, the more tightly associated the variables are (the closer the dots will fall on a line). 0.9 is very high (very close to 1), so the points will fall near/on the line of best fit.
  17. C – Acute abdominal pain in a fertile woman is very frequently going to be an ectopic question. Associated fun fact: the most common cause of secondary amenorrhea is pregnancy.
  18. B – Schizophrenics take antipsychotics which can (extremely rarely) cause neuroleptic malignant syndrome (NMS): rigidity, confusion, fever, autonomic instability. A related but distinct scenario is “serotonin syndrome,” which can be caused by a variety of antidepressants and related compounds. The clinical presentations share some similarities, so the medication history is particularly helpful.
  19. E – The chromosome appears truncated/small, so something has been deleted. If something lost leads to cancer, then it is the loss of a tumor suppressor.
  20. A – Esophageal dilation that tapers at a non-relaxing lower esophageal sphincter (the so-called “bird’s beak” appearance) is classic achalasia.
  21. B – HGPRT = high uric acid. Self-mutilating behavior (e.g. bad finger biting) is one of the more specific clinical features.
  22. E – Those are sickle cells on the smear. LUQ pain on test questions almost always means splenic pathology. All sickle cell kids will eventually infarct their spleen.
  23. E – Profuse super-watery diarrhea means cholera (the so-called “rice water stool”). That said, regardless of the cause, you treat all causes of volume loss with volume replacement (normal saline)!
  24. D – Tamoxifen is a selective estrogen receptor modulator (SERM). It blocks estrogen receptors in the breast (good to prevent breast cancer recurrence), but it actually activates uterine estrogen receptors and increases the risk of endometrial hyperplasia and endometrial carcinoma. Abnormal vaginal bleeding in a patient over the age of 35 should prompt an endometrial biopsy to rule out cancer.
  25. D – Learning how to interpret hepatitis panels is high yield. This patient has the Hepatitis B surface antigen (that the virus is making) but does not yet have the antibody against this antigen that confers immunity. Generally, antibodies to surface antigens are protective.
  26. C – Classic Wernicke’s encephalopathy: confusion, jiggly eyes (ophthalmoplegia), and ataxia. It’s thiamine deficiency (same deficiency in beriberi). Add in anterograde amnesia and confabulation and you have a class USMLE presentation of Korsakoff’s psychosis (Wernicke-Korsakoff syndrome).
  27. E – Intermittent unilateral throbbing/pulsatile headaches are migrainous, especially if associated with nausea/vomiting, photophobia, phonophobia, etc. Triptans are drugs of choice for the treatment of acute migraine episodes. Tricyclic antidepressants (TCAs) like amitriptyline are used for migraine prophylaxis. Oxygen is a treatment for cluster headache.
  28. C – Common sense is key, particularly for counseling-type questions. Patients have autonomy and can do whatever they want; it’s your job to explain the risks and benefits. The patients ultimately make their own treatment choices.
  29. H – It’s okay to be normal, especially on developmental and psychiatry questions.
  30. D – You need to memorize the list of drugs that induce and inhibit CYP450. Warfarin has oodles and oodles of drug reactions for this reason with potentially dire consequences.
  31. C – The only reasonable choice is always the correct one. Address patient concerns and respect their autonomy, even if they seem silly.
  32. D – ITP causes immune-mediated consumption of platelets, hence the low platelet count and petechiae. The bone marrow biopsy results demonstrate that the body has appropriately increased platelet production, meaning that this is not a platelet production issue. While TTP has a similar acronym, it’s an entirely different disease with a classic pentad: thrombocytopenia (low platelet count), microangiopathic hemolytic anemia, altered mental status, renal failure, and fever.
  33. C – Middle-aged woman with progressive shortness of breath? Think of idiopathic pulmonary fibrosis, a restrictive lung disease.
  34. A – A mix of upper (hyperreflexia, babinski) and lower (fasiculations, atrophy) motor neuron signs means ALS. Know your UMN/LMN signs.
  35. B – Low B and T cells means severe combined immune deficiency (SCID), which is caused by adenosinde deaminase deficiency. Know your immune deficiencies.
  36. A – Pyknosis and nuclear fragmentation are part of the process of apoptosis.
  37. E – Subacute combined degeneration (progressive peripheral sensory and motor loss) is a late sign of B12 deficiency, which is common in old people. On board exams, a geriatric patient who lives alone and may have a “tea and toast” diet is likely to have vitamin deficiencies, particularly of folate and B12.
  38. E – You can often ignore the pictures. Endotracheal tubes irritate the sensitive respiratory mucosa, causing the more delicate pseudostratified ciliated epithelium to be replaced by metaplastic stratified squamous epithelium (the tissue most suited to areas of constant abrasion). When tissues are exposed to an atypical/noxious environment, they can become metaplastic as an adaptation (just as in Barrett’s esophagus, when esophageal stratified squamous tissue becomes simple columnar epithelium [like stomach/bowel] after prolonged exposure to acidic gastric secretions.)
  39. A – Phenylephrine is an alpha-agonist nasal spray and oral medication that often shows up on exams. Oxymetazoline (Afrin) is a similar alpha-agonist nasal spray. Pseudoephedrine is another decongestant with a similar MOA that’s less common now that it’s a federally monitored component of methamphetamine production. Topical alpha agonist decongestants are physically addictive and can cause miserable rebound congestion (rhinitis medicamentosa). Systemic formulations can cause hypertension and worsen prostate problems by causing prostatic smooth muscle constriction. Just remember that tamsolusin (Flomax) is an alpha-5a antagonist, which relaxes prostate muscle.
  40. B – Memorize aspirin’s acid-base effects: metabolic acidosis and respiratory alkalosis. Note, this is actual respiratory alkalosis, not simply normal respiratory compensation for metabolic acidosis.
  41. C – Prolactin is elevated (hence the amenorrhea and galactorrhea). Your first thought is a pituitary microadenoma, which isn’t an answer choice. Then you remember that dopamine is also known as “prolactin inhibiting hormone” and that a lesion of the infundibular stalk would prevent dopamine released by the hypothalamus from reaching the pituitary gland to inhibit prolactin secretion, thus causing hyperprolactinemia. That’s a mouthful.
  42. A – Turner syndrome (you may remember lymphedema of the neck by another name: cystic hygroma). The 45,X gives it away though anyway. Mosaic Turner’s syndrome and the mosaic trisomies result from nondisjunction during mitosis. In total monosomy/trisomies, the cause is nondisjunction during meiosis. Uniparental disomy is essentially only tested via the Prader-Willi and Angelman syndromes (chromosome 15).
  43. C – GVHD sucks. Skin and GI lesions are especially common sites (mucosal tissues are rapidly dividing and thus prone to attack).
  44. C – It’s just anatomy. What hip flexor is up in the abdomen near the appendix?
  45. E – Hypertension, headache, visual changes, etc during pregnancy mean preeclampsia. Once you get a seizure, it’s eclampsia. Treatment is rapid delivery of the fetus (with magnesium to prevent seizures).
  46. B – Azotemia, hemoconcentration, and hypotension/tachycardia are all results of dehydration. This may lead to acute renal failure if allowed to continue (but this diagnosis requires a change in creatinine of 0.3).

Block 2

  1. C – The renal failure means that NSAIDs, methotrexate, and colchicine are all undesirable (all can cause or exacerbate renal failure). Etanercept is a TNF-antagonist and all-around DMARD (disease-modifying antirheumatic drug) used for RA, psoriasis, etc. The main common testable side effect of all DMARDs is immune-suppression. The rare (but still testable) complication is leukemia/lymphoma.
  2. C – When patients are nurses in test questions, it’s a tip-off (and sometimes a fake-out) that they have factitious disorder aka Munchausen’s (because they have both the access and knowledge to obtain and use medications). The way you tell an insulinoma from exogenous insulin administration is via c-peptide (the cleavage product of endogenous insulin). If c-peptide and insulin levels are both high, then it’s an insulinoma (most commonly located in the pancreas). If c-peptide is disproportionately low, it implies exogenous insulin supplementation.
  3. E – Athletes are another red flag for self-inflicted conditions, particularly dancers, gymnasts, wrestlers, and other athletes who have to weigh in. Urine excretion of sodium and potassium are inappropriately high given the hypokalemia, because the diuretics she is abusing (to lose weight) are forcing renal excretion. Aldosterone deficiency (aka RTA type 4) causes hyperkalemia, not hypo.
  4. D – It’s important to know the psychiatric manifestations of medical conditions. Hypothyroidism can cause depression. The slow pulse, thinning hair, and weight gain are all distinct from simple major depression.
  5. B – Crossed findings means a brainstem lesion. Left (ipsilateral) tongue, right-sided (contralateral) weakness means the exiting left hypoglossal nerve has been affected (within the left medulla). B is in the pyramid where the corticospinal tract runs to control muscles (prior to the decussation). This is known as medial medullary syndrome or Dejerine syndrome.
  6. D – Recurrent respiratory infections could be a lot of things. But then they mention the dextrocardia. Kartagener syndrome is the combination of situs inversus and defective cilia (due to a mutation in dynein), where the inability to effectively clear secretions results in recurrent sinusitis and bronchiectasis.
  7. B – Fever and acute left lower quadrant pain in an older person is always diverticulitis on exams. In a younger woman, you need to also consider ovarian torsion, ovarian cyst rupture, pelvic inflammatory disease/tubo-ovarian abscess, and ectopic pregnancy. Otherwise, diverticulitis. The association on tests is just as strong as RLQ pain and appendicitis. Remember: diverticulitis hurts, diverticulosis bleeds.
  8. E – Delayed separation of the umbilical stump is a classic clue for leukocyte adhesion deficiency. ICAM is the defective ligand most often asked about as a second order question.
  9. B – Aminoglycoside antibiotics (gentamycin, amikacin, etc) are powerful antibiotics especially useful for bad gram-negative infections. Bad side effects are permanent hearing loss (ototoxicity) and renal failure. Both are important to know. Torsades de Pointes (choice E) can be caused by fluoroquinolones (e.g. cipro), as these drugs prolong the QT-interval.
  10. E – Don’t be scared by acid-base. Figure out the primary disturbance first, then the compensation follows. If you’re vomiting all the time, you’re losing acid. In order to produce more acid in your stomach, there has to be a kickback of bicarb into your bloodstream (H20 + C02 = H + HCO3). So primarily you have a metabolic alkalosis: high pH and high HCO3. Then you have respiratory acidosis (high PC02) to compensate.
  11. B – Gram-negative bacteremia causes sepsis via endotoxin (lipopolysaccharide, a component of the outer membrane of gram-negative bacteria). Superantigens (choice E) are exotoxins (like those produced by staph and strep)
  12. G – OCD is treated first with SSRIs.
  13. E – Basic anatomy. You should probably remember that the SVC and IVC are on the left, as is the azygous vein. On CT scans, black means air, so C + D are the right and left mainstem bronchi.
  14. C – The hardest part of many of behavioral/psychiatry questions is knowing when something has crossed from normal into pathology. In this case, the boy is 8 years old (several years after he should have gotten over going to school) and his separation anxiety is getting in the way of his functioning (he’s missing school). Function is the key between normal and abnormal.
  15. C – This is the Arthus reaction, a classic Type III hypersensitivity. Type III hypersensititives are due to circulating antibody-antigen complexes. In this case, the booster vaccine antigen is bound to the preformed antibodies which then become lodged in the dermal blood vessels, leading to local damage and in some cases ulceration/necrosis. Type I is near instantaneous histamine release and would occur hours after the injection (and wouldn’t lead to skin necrosis). Type II  hypersensitivity reactions are due to preformed circulating antibodies formed by the initial exposure which then bind to antigen present on cells (like RBC surface, glomerular basement membrane) which lead to cell lysis and local damage. Type IV (T-cell mediated) reactions are like a PPD; they take a while.
  16. B – One drink may be good for heart health, but habitual abuse can cause alcoholic cardiomyopathy. Everything else is good for you but moreso for reducing the risk factors for atherosclerosis (which leads to myocardial infarction and ischemic cardiomyopathy). In this case, his coronary arteries are clean.
  17. E – All of the congenital heart defects and their associated murmurs are high-yield and worth memorizing. Understanding murmur physiology is also high yield in general. In this case, we have a cyanotic heart condition in a newborn. Tetralogy of Fallot is the most common cyanotic heart lesion on tests and in real life. ASDs–fixed split S2–are left-to-right (non-cyanotic lesions), at least until they reverse down the line (Eisenmenger syndrome). Clinically significant PDAs are alluded to by their continuous “machinery” murmur.
  18. A – Just remember that T (thymine) in DNA equals U (uracil) in RNA (which is what is used to actually make the protein). This is actually obvious based on the way the chart is given: CAG are present but T is absent, so via process of elimination…
  19. C – Anaphylaxis is treated with epinephrine.
  20. D – Some patients with rheumatic fever end up with rheumatic heart disease, which results in stenosis (stiffening) of the mitral valve (and less commonly the aortic value). Mitral valve regurgitation, on the other hand, is often seen secondary to progression of mitral valve prolapse or—more dramatically—after papillary muscle rupture as a complication of MI.
  21. E – If you want to drain fluid, you have to be low enough in the chest to get it, but not so low that you’ll puncture through the diaphragm into the abdomen.
  22. E – Lymphedema is a common surgical complication, particularly when extensive lymph node dissections are involved (e.g. abdominal tumor debulking for ovarian cancer, axillary exploration for breast cancer). DVT, the other thing you’re thinking of, is typically more acute, red, swollen, painful on exam questions, but—more importantly in this case—causes swelling primarily via increased capillary hydrostatic pressure (not an answer choice!)
  23. B – The other choices actually aren’t in this location, so they don’t make sense. But the actual testing point here is that people with diabetes get horrible atherosclerosis, particularly of the small to medium size vessels not as commonly affected by hyperlipidemia and smoking.
  24. A – Mastitis and breast abscess are common, particularly during breastfeeding. The other thing with redness and skin changes that you’re worried about is inflammatory breast cancer, but note here that the skin is firm and dimpled, not erythematous. And the patient is 24. Peau d’orange seen with lymphatic obstruction due to breast cancer is rare and would not be focal in the area of a prior infection.
  25. F – Traveling to a faraway land is a clue to think of something a little more fascinating than the usual. Unicellular organisms that eat red blood cells and mucosal ulcers (dysentery, pain, etc)? That would be amebiasis due to Entamoeba histolytica, which thrive in dirty water (fecal-oral route of transmission). This is also the organism that causes amebic liver abscesses, another test favorite. Snail-contaminated water (choice G) leads to schistosomiasis. Malaria parasites (from mosquitos, choice C) don’t eat red blood cells; they infect red blood cells.
  26. E – This a nice list of the physical exam findings in pneumonia.
  27. B – The patient is in diabetic ketoacidosis (DKA). The key to the question is straight up biochemistry: insulin stimulates phosphatases and inhibits kinases. Protein kinase A phosphorylates/stimulates adenyl cyclase which increases the production of cAMP. So without insulin: high kinase, low phosphatase, high cAMP.
  28. A – The erectile dysfunction drugs like sildenafil inhibit phosphodiesterase type five (PDE5), which relax smooth muscle (hence the orthostasis). Prazosin, doxazosin, and (the more selective) tamsulosin, which are drugs used to reduce symptoms from prostate hypertrophy, are alpha blockers.
  29. A – She has nephrogenic diabetes insipidus (large volume dilute urine production that cannot be reversed with the administration of exogenous vasopressin). Aquaporins are the water channels that allow for the reabsorption of free water from the collecting ducts and the production of concentrated urine.
  30. A – Of the choices provided, only two are biologics. Anakinra blocks IL-1. Adalimumab (Humira) blocks TNF-1 (another testable TNF-antagonist is etanercept). These drugs are worth knowing.
  31. B – Six-year-olds typically understand the finality of death. Infants have no understanding, whereas preschool-age children often think of deaths in reversible or metaphorical terms.
  32. A – All the vesicles contain the same viral infection, so all should have the same appearance on gel.
  33. A – AZT (zidovudine), the first reverse transcriptase inhibitor, is associated with anemia secondary to red cell aplasia. Reactivation of parvovirus B19 infection in HIV patients after treatment can also cause aplastic anemia.
  34. E – No state-mandated screening you say? Hint hint. PKU. Another common disease in the “missed the state screen” questions is congenital hypothyroidism (sometimes still awkwardly called “cretinism”).
  35. B – Alcoholics (and any person with an altered level of consciousness) are a set-up for aspiration pneumonia: classically RLL, classically foul-smelling. The organism on tests will be Klebsiella.
  36. D – Massive proteinuria in children is almost always minimal change disease, so-called because the glomeruli look normal on conventional microscopy (podocyte “foot process” fusion is seen on electron microscopy). Big time swelling ensures as serum albumin leaks into the urine and plasma oncotic pressure decreases. Children respond well to treatment compared with adults, and the disease is usually reversible with steroid treatment. Viral infection is a common instigator, as is NSAID use.
  37. B – Acute onset of altered mental status (paranoia, confusion, etc) is delirium, which often carries a waxing/waning course (e.g. seems great on rounds, decompensates at night). The elderly are particularly vulnerable. Dementia is a chronic, progressive condition. Note, delirium is frequently superimposed on a baseline of dementia.
  38. B – Gout, gout, gout. Allopurinol helps prevent flares but does nothing to treat them. Treatments of choice for an acute flare are NSAIDS or colchicine.
  39. A – Renal failure causes metabolic acidosis, hence the low bicarb.
  40. C – The meningitis diagnosis is a giveaway. Your job is to know two things: 1) the common pathologic organisms in different age groups or 2) that Neisseria is a gram-negative diplococcus.
  41. E – The whole afferent/efferent thing is worth knowing. When the efferent arteriole is independently constricted, the blood can get into the glomerulus but has difficulty getting out. So more blood spends a greater amount of time in the glomerulus being filtered: GFR up, filtration fraction up, but overall blood flow is decreased due to the increased resistance of the system as a whole.
  42. E –  Tons of exercise followed by renal failure is always going to be rhabdomyolysis. Dark urine (without actual blood in it) is due to myoglobinuria, which can cause a false positive urine dipstick.
  43. D – The most common tumor of the parotid gland by far is the pleomorphic adenoma. These are not aggressive but are difficult to resect due to the important surrounding real estate (the facial nerve branches), so local recurrence is common.
  44. C – If an adult is taking on behaviors common to children, it’s called regression (even if it’s something you might do yourself; this isn’t a judgmental thing).
  45. C – If you take in less food, your body has to use its stores. Lipid oxidation is how we “burn fat.”
  46. E – Tetracycline-use during childhood causes classic tooth discoloration either referred to as brownish/yellowish or grayish.

Block 3

  1. A – You can rule out femoral, obturator, and sciatic immediately because these travel in the pelvis and thigh—far too high to be affected by a lower extremity fracture below the knee. Foot drop and dorsal/lateral sensation relate to the peroneal/fibular nerve. The tibial nerve, in contrast, innervates the calf muscles.
  2. A – Ballet dancers (wrestlers, models, ballet dancers, athletes who need to make weight, and particularly young women in general) all have eating disorders on Step 1. Folic acid is involved in the production of both red and white blood cells. Remember, low folate leads to macrocytic anemia and hypersegmented neutrophils. B12 deficiency (not an answer choice) leads to macrocytic anemia and neurological changes (including SCID in severe cases).
  3. B – This patient has cystic fibrosis. The combination of respiratory and GI issues is classic and caused by ineffective chloride transport and consequently thick exocrine secretions, which clog up the airways and the pancreatic ducts.
  4. D – The suprachiasmatic nucleus of the hypothalamus controls circadian rhythms. A few more key thalamic nuclei are worth knowing: Supraoptic releases vasopressin (ADH). The lateral nucleus controls thirst and hunger. The ventromedial controls satiety. Anterior controls temperature. The paraventricular nucleus release CRH, TRH, and oxytocin.
  5. D – Malaria infections caused by Plasmodium vivax are prone to relapse due to this organism’s ability to infiltrate and then lie dormant in the liver.
  6. B – You are seeing a tube inside of a tube in the pathology specimen. Yes, look again: bowel within bowel. Intussusception. Currant jelly stools. Now, a right lower quadrant lesion that causes pain and hematochezia, particularly in a child? Then you get to think of Meckel’s (with its rule of 2’s).
  7. D – “Functional defects” is their sly way of saying that the “oxidative burst” in his neutrophils isn’t exactly working, because this patient has chronic granulomatous disease due to NADPH oxidase deficiency. Recurrent infections with pus and abscess-forming infections like Staph are a hallmark. The second sentence is there to convince you that his B and T cells work just fine.
  8. B – This patient has symptomatic anemia. Its microcytic nature implies iron deficiency, which is most commonly due to occult blood loss. In the elderly, the concern is colon cancer. In a reproductive age female, iron deficiency is more commonly secondary to uterine pathology.
  9. D – Mineralocorticoid equals fludrocortisone equals aldosterone analog. Now, you know that he’s going to be retaining the fluid he’s receiving. Remember the #1 rule of renal physiology: water follows salt. Looking at the chart, he retains most of the salt for the first two days and then excretes almost all of it the third and fourth days. So he takes in 800 mmol NaCl and excretes 500 mmol (30 + 90  + 180 + 200). So he’s 300 mmol up, which corresponds to 2 kg (based on the weight for normal saline they’ve provided). On a side note, this is as hard as the math gets on Step 1.
  10. D – Cholesterol xanthomas, horrible serum cholesterol levels, and early death by MI are all signs of familial hypercholesterolemia (caused by a defect in the LDL receptor).
  11. D – Those are varicose veins, and they’ve described symptomatic varicosities. Incompetent valves allow reflux of blood into the dependent feet and legs. The pooling blood increases hydrostatic pressure, causing edema.
  12. E – High conjugated bilirubin means that the liver is working. Acholic (pale or clay-colored, not brown) stools, however, mean that this conjugated bile is not being excreted normally into the small bowel. So there’s a block (obstruction). A congenital block is typically due to either a choledochal cyst or biliary atresia.
  13. B – When you see a therapy question, your default answer will always be CBT and you will nearly always be right. It’s been shown to be effective as an evidence-based treatment for a variety of psychiatric conditions.
  14. G – Sulfonylurea medications (glipizide, glyburide) stimulate the pancreas to secrete more insulin. For this reason, they are most efficacious early in the disease process when pancreas still has remaining functional reserve.
  15. E – Sensitivity and specificity are the most tested stat questions by far, probably followed by positive and negative predictive values, and incidence vs. prevalence. The application of statistics on the boards is limited and very approachable. For an intuitive understanding, specificity is how good a test is at ruling a disease in; sensitivity is how good a test is at ruling a disease out. Specificity, in, sensitivity out (SPIN & SNOUT as the mnemonic goes). A screening test needs high sensitivity (HIV ELISA test). A confirmatory test needs high specificity (HIV western blot). Sensitivity is True positive / (True positive + False negative) = 60 / (60 + 20) = 0.75.
  16. B – Kinesin is the motor protein responsible for anterograde microtubule intracellular transport used in mitosis, meiosis, and axonal transport (dynein is retrograde). In this case, kinesin is responsible for bringing synaptic vesicles produced in the nucleus down to the axon terminus.
  17. A – It’s the antibodies to surface antigens that are protective. Hemagglutinin is a surface antigen that is responsible for clumping RBCs in vitro.
  18. D – This patient has multiple myeloma, a plasma cell cancer. Nontraumatic bone pain with multiple lab abnormalities is key. Lytic lesions are seen on radiographs. Anemia is secondary to bone marrow infiltration by plasma cells. Hypercalcemia is due to bone destruction. Proteinuria (and eventually renal failure) is due to abnormal light chains (Bence Jones proteins) plugging up the kidneys.
  19. B – When the patient has obvious motives, then secondary gain is a more likely explanation over a primary psychiatric disorder. A hardened violent criminal who threatens his physicians to get what he wants? Sounds antisocial to me too.
  20. D – Leukocoria (white pupillary reflex) in a newborn can (rarely) be a sign of a retinoblastoma (and on Step 1, it’s always a sign of retinoblastoma). The defective tumor suppressor protein, Rb1, normally prevents the transition from G1 to S phase, where DNA replication occurs.
  21. C – The boy and maternal uncles are affected? Sounds X-linked. Coarse facial features and large testicles? Sounds like Fragile X. But the diagnosis doesn’t matter, only the genetics. The mother passes down 1 of her 2 X chromosomes to her daughters. The other X comes from Dad. So half of the daughters will be carriers. (By the same token, half of the boys will have the disease).
  22. B – Choriocarcinoma is a much-feared complication of a molar pregnancy. It is a cancer of the bHCG-producing syncytiotrophoblasts found in the placenta.
  23. C – That’s a litany of symptoms with only one reasonable single possible cause out of the provided choices: hyperthyroidism. Elevated thyroid hormone can manifest as anxiety, GI hypermotility, tachycardia and a-fib, weight loss, heat intolerance, etc. Thyroid disorders are very high yield. Pheochromocytomas (choice E) can cause some of the same symptoms in an episodic fashion (more typically panic attacks, episodic severe hypertension, headache).
  24. B – Finasteride (aka Propecia) is used for male pattern baldness and prostate hypertrophy. It’s a 5-alpha-reductase inhibitor, which prevents the conversion of testosterone to dihydrotestosterone (DHT).
  25. A – The most common cause of hypothyroidism in developed countries is Hashimoto’s thyroiditis. In the developing world, it’s iodine deficiency. Hashimoto’s disease is typified by glandular enlargement and sometimes overt clinical hyperthyroidism as the autoimmune attack commences, followed by gradual atrophy and hypothyroidism as the gland is destroyed.
  26. C – Amphotericin B is terrible for kidneys. A liposomol amphotericin formulation now exists that is better tolerated but quite pricey.
  27. B – If there is inflammation, a steroid will decrease it. In this case, we have giant cell arteritis (GCA) aka “temporal arteritis,” a potentially blindness-causing autoimmune vasculitis often associated with polymyalgia rheumatica.
  28. H – Furosemide is the prototypical “loop” diuretic, which works by blocking the triporter and preventing the reabsorption of 1 K, 1 Na, and 2 Cl ions. So less K/Na/Cl ion transport causes decreased osmolarity of the medullary interstitium (where these ions would normally enter). Water follows solute, so with fewer ions reabsorbed, less water will be reabsorbed.
  29. C – RSV, like all respiratory viruses, spreads via respiratory droplet. Babies are too young to wheeze because of asthma; they wheeze because of RSV.
  30. B – She has temporal mandibular joint syndrome/disorder, simply called TMJ by laypeople everywhere. This question is actually asking you if you know what the muscles of mastication are: masseter, temporalis, and the medial and lateral pterygoids (all innervated by the mandibular branch of V).
  31. D – Volume resuscitation is always with normal saline. Anything else given in quantity could cause undesirable electrolyte shifts.
  32. A – An ulcer that makes you cry? That would be chancroid, caused by H. ducreyi (that rhymes, more or less). The painless big ulcer caused by syphilis (treponema pallidum, choice D) is called a chancre.
  33. B – Clozapine is famous for its association with life-threatening agranulocytosis. In fact, this is so feared that patients who begin this medication must have weekly blood draws for several months to monitor for it. It’s also on every test ever.
  34. E – This patient has SIADH causing hyponatremia. When you think of the paraneoplastic syndromes of lung cancer, you should generally think of small cell (aka “oat” cell) carcinoma, as it has several: SIADH, Cushing’s syndrome (due to tumor ACTH production), and Lambert-Eaton myasthenic syndrome (weakness caused by presynaptic calcium channel antibodies). Squamous cell carcinoma, on the other hand, is more typically associated with hypercalcemia (the hard “q” in squamous sounds like the “Ca” in calcium if you ask me).  That said, a variety of lung and brain lung pathologies can cause SIADH, not just small cell.
  35. A – Hirschprung’s disease is caused by a failure of neural crest migration and subsequent absence of the myenteric Auerbach’s plexus in the distal colon. The classic bird’s beak appearance is what the question is describing.
  36. E – By far the most common cause of sudden death in patients after MI (and with severe structural heart disease) is ventricular fibrillation. In v-fib, the ventricles are completely uncoordinated, providing no meaningful stroke volume for perfusion. Nothing else on that list can cause you to literally drop dead.
  37. A – Acetaminophen (Tylenol) can cause fulminant hepatic failure in overdose. This will probably be on your test.
  38. C – Pneumonia that arises after a convention (typically old people in a hotel or on a cruise ship etc) is due to Legionella, which can colonize water supplies and infect large numbers of people. Shiny opal-like colonies on yeast extract is almost always a component of the test question.
  39. D – This is rheumatic fever from group A strep pharyngitis. This is thankfully rare now, as we routinely treat Step throat with antibiotics. The cause of all the damage is due to cross-reactivity of Strep antigens with the tissues of the heart, joints, skin, and brain. Anti-streptolysin O (ASO) and anti-DNase titers will be high.
  40. H – Differentiating between ulcerative colitis and Crohn’s disease is a Step 1 favorite. In real life, differentiation can be difficult, but on tests, ulcerative colitis is marked by continuous mucosal involvement starting in the rectum and extending proximally. CD is marked by transmural involvement with skip lesions that can extend from mouth to anus. CD is more likely to be complicated by fistula and abscess formation. UC is more likely to be associated with extra-GI symptoms such as anterior uveitis, as well as a greater risk of colon cancer.
  41. C – In the short term, grief can be quite dramatic. Thoughts of guilt and overwhelming sadness or common. Phrases like, “I wish I could be with him/her” etc are not pathological, but actual suicidal ideation with plan/intent, weight loss due to depression-related anorexia, etc all suggest pathology.
  42. B – Osteogenesis imperfecta (blue sclera, lots of fractures [they even occur prenatally]) is a defect in type 1 collagen.
  43. C – Gaucher disease is one of the list of inherited lysosomal storage diseases that you will memorize every few days while studying and then forget instantaneously. Gaucher disease is caused by a defect in the enzyme glucocerebrosidase, leading to an accumulation of glucocerebroside. The handicapped reticuloendothelial cells cannot fully perform their housekeeping functions, and the compound accumulates causing foamy macrophages (crumpled tissue appearance), hepatosplenomegaly, ± neurological symptoms and mental retardation.
  44. A – Paternalism is when a physician makes decisions for patients instead of providing them with options and allowing them to then make an informed decision. It’s old school.
  45. E – Basic hand anatomy. To answer this particular question correctly, you don’t actually need to know what structure travels in between these tendons (it’s the recurrent branch of the median nerve). You would need to remember, however, that the radial side is lateral (the thumb side), the ulnar side is medial (pinky finger side), and the fingers (digitorum) are in the middle. So the cut is towards the thumb side, which only one answer choice includes. And, if you remember the distribution of the median and ulnar nerves, the rest of the answers don’t make any sense.
  46. F – While staph aureus is the most common cause of hematogenous osteomyelitis in children, Salmonella is the most frequent cause for patients with sickle cell anemia (a test favorite).

Requests for further clarifications etc can be made in the comments below.

You may also enjoy some other entries in the USMLE Step 1 series:
How to approach the USMLE Step 1
How to approach NBME/USMLE questions
Free USMLE Step 1 Questions
The newest (2014-15) USMLE practice material explanations


cy 03.21.14 Reply

hey ben,

thanks for this post. it’s good get a brief summary of main ideas for these questions.

could you explain why block 2 (48) the diagnosis is insulinoma when the insulin is at the upper range of normal but the c-peptide is at the lower range?

Ben 03.22.14 Reply

The lab tests aren’t really the crux here, the question is really specifically asking you where tumors that cause hypoglycemia live. The insulin level and c-peptide are concordant, so we’re not being led to consider factitious disorder or anything. The gut instinct is to believe that insulin must be sky high to diagnose an insulinoma, but this is not the case. How dramatic the hyperinsulinemia is depends on the size of the tumor, its physiologic activity, and when the labs are drawn relative to the episode. Factitious disorder, on the other hand, would be suspected in a situation when insulin is high but c-peptide is disproportionately low. Upper range vs lower range isn’t worth splitting hairs over, as these are both ranges—the point is that they aren’t significantly different.

Also note that in this case, factitious disorder isn’t even a possible answer choice (they would probably make the labs more obvious if it were). The point is to know which tumor causes episodic hypoglycemia and where its most common location is.

cy 03.25.14 Reply

cool, thanks!

Annabel 04.14.14 Reply

Thanks for the post! Very helpful

Diane 05.01.14 Reply

Thank you! Especially loved the comment about Gaucher disease… ;)

Ben 05.02.14 Reply

You’re welcome. I always try to be as accurate as possible!

Charlie 05.03.14 Reply

Thanks for posting these! I just took the practice test at prometric and then came home and went over the materials. I used your explanations; they were really helpful!

Anonymous 05.15.14 Reply

Thank you!!! SUPER helpful!

Ben 05.16.14 Reply

Glad to hear it!

sdn user 06.08.14 Reply

Hi Ben,

Saw your posts on SDN and went through all of these. Very helpful so thanks!

I also don’t really understand Block 2 number 48’s notorious insulinoma question. Sorry to bring it up again, read the above comment as well. I get that C-pep and Insulin are secreted together in normal circumstances, as well as in insulinoma, as opposed to factitious insulin administration (Cpeptide below 0.5 and insulin somewhere in the normal 5-20 or higher range?). But here the insulin is normal (ok i get that it can be normal in an insulinoma) but the c-pep is LOW NORMAL (0.5-2.5 is normal, it’s at 0.5) – shouldn’t it be closer to 2.5 if it’s an insulinoma? Also is Cortisol low?(10 ug/dl when normal is “0800 h: 5-23 μg/dL // 1600 h: 3-15 μg/dL 2000 h:<50% of 0800 h"). Not sure what to make of those given values, I know Cortisol varies based on ACTH which is high in the AM I believe.

Also for Block 3 number 98's intusseception question – how can i tell this apart from a fecalith obstructing the appendix? I mean…that is the appendix right (the little finger like thing)? I guess if it were appendicitis, the fecalith would be IN the appendix and not outside of it and the appendix would be inflamed? It's just that the google pics of intuscception all look shiny and less "solid" if that make sense — the only google pic i could find of what i view as "solid looking" is like this https://www.flickr.com/photos/jian-hua_qiao_md/8188944720/

Lastly, on Block 3 number 120 about the loops — I was thrown about the last column "water absorption in the DESCENDING loop of henle" which is before the Thick ascending limb where loops work. So are we talking about the water absorption in the Descending Limb before or after steady state? Wouldn't there be increased absorption of water if we were losing a lot of water with loops?

Thanks in advance!

Ben 06.08.14 Reply

Glad they’ve been helpful.

48) You would think C-peptide should be equivalent to insulin (high-normal and high-normal), but lab tests actually aren’t that magically reliable. Too many factors at play in a real live physiological patient. For one, the half-lives of the two are different. I agree that the labs here aren’t particularly suggestive of an insulinoma (if it were a factitious disorder vs insulinoma question, they would be obvious). This question is only asking where do tumors that cause hypoglycemia live.

98) That’s actually not the appendix. The picture is a little weird because they’ve sliced open the bowel, so what you are seeing is a loop of small bowel and some necrotic tissue and thrombus which is lying on the bed of the bowel that it telescoped into. As with almost all picture questions, the question can be answered without use of the image. The clinical picture (rapid onset pain, hyperactive bowel sounds, palpable mass) is classic intussusception. Appendicitis is never going to come at you like that. Also, the appendix is usually less than 8 mm. The specimen you are looking at is like 3 cm (the size of small bowel).

120) This is how the countercurrent exchange mechanism works. Ions are pumped into the medullary interstitium in the ascending loop, but the increased osmolarity then causes increased water absorption in the descending loop, allowing for the creation of increasingly concentrated filtrate. Resorption of water in the limb depends on having a hyperosmotic medullary interstitium, which is created via the triporter. With lasix, you do have increased free water absorption distally in the collecting ducts, but this compensation is not able to keep up with the solute tide and thus diuresis ensues.

SDN user 06.08.14 Reply

Thanks Ben! Really appreciate having your taking the time for this! I think I forgot about the countercurrent multiplication :( If you have some time over the next 2 days (my test is on wednesday), do you think you could please do an update of the answers of the new 2014 Free “150” questions? It’s really more around 71 new questions b/c there are lots of repeats. Here is the link: http://www.usmle.org/pdfs/step-1/2014samples_step1.pdf

Of course if you can’t I understand too that you’re very busy. Thanks either way!

Ben 06.09.14 Reply

I do plan on doing an update for the new question set, but sadly they won’t be ready in time for your test (or likely anyone taking the test in the next couple of weeks).

impu 06.20.14 Reply

Hey Ben…Great. Thanks a lot.
I am looking forward for the explanation for the release of 2014 nbme/fred 150 questions.
thanks .

Ben 06.30.14 Reply

The explanations for the 2014-2015 Official USMLE practice materials have finally arrived. You can find them here.

I may have mismarked a few in counting, but I believe the majority (85 of the 138) are new. Therefore if you were to do both the 2013-14 and 2014-15 sets, you would complete around 223 questions (with explanations, of course). Not bad.

GBUTL 07.01.14 Reply

I really like the way you think.

Ben 07.15.14 Reply


mc 10.22.14 Reply

Considering that you took the time to help others like me, i’d like to take the time to tell you how much help you’ve been. thank you so much for explaining the answers to us.

Ben 10.23.14 Reply


jat 01.12.15 Reply

Hi Ben
I have been working on the 2013-14 Step-2 practice MCQ’s and realise your ansewr’s and explanations are for the 2014-15 questions. Do you have an explanation sheet for Step-2 2013-14 practice MCQ’s. I am working out the questions for myself but your explanations are very helpful.

Ben 01.12.15 Reply

I don’t, sorry. I may do an update though when the next set comes out later this year.

They switched over to the 2014 set earlier in the year for Step 2 than for Step 1, which is when I started writing out the explanations. So 2013-14 was only the current set for Step 1 at the time of writing, which is why there are two sets for Step 1 but only one for Step 2 CK on this site at the moment. If there are any ones in particular that were bothering you, feel free to post those question numbers on the Step 2 CK question page and I can try to help out.

jat 01.13.15 Reply

Thanks that is fine, I think I downloaded my questions from USMLE forum and they were not the latest set. I think most of the explanations for 2013-2014 are in the current set you have posted just a different order, so I have been looking through them and finding what I need. Thanks for your reply.

AS 01.21.15 Reply

Dear Ben,

Many thanks for this – very useful.

I noticed that for Q61, you mentioned type II hypersensitivity. But don’t you think that the question is describing Arthus reaction and thus a type III hypersensitivity (i.e., antibodies + complements are involved)? It gives us the same answer, but my question is with the explanation you provided. Maybe I’m wrong.

Thank you,

Ben 01.22.15 Reply

You are absolutely correct. That’s an oversight/poor explanation; I had described the three other hypersensitivities/choices without explaining the type III. It’s not a type II hypersensitivity, which are direct by antibodies binding to the body’s own cells due to some presented antigen and mediating damage (this is how a penicillin allergy works). In type III reactions, it’s not the cells with are attacked by antibodies; it’s the antigen-antibody complexes which circulate around and become deposited, often in small vessels, such as the Arthus reaction, in which plugging and inflammation of dermal blood vessels can cause skin ulceration and necrosis.

To be clear though, both type II and type III involve antibodies and complement. Type II hypersensitivities typically generate the membrane attack complex as the mechanism of damage. The distinguishing characteristic is that type III are free floating antigen-antibody complexes and type II are related to antigens which are presented by/attached to a cell.

Thanks for the catch; it’s fixed now.

Sam 04.25.15 Reply

Thank you so much for creating this blog, I really appreciate it! I just had a question on number 84 – the question states that the crystals are “birefringent” but wouldn’t gout crystals exhibit negative birefringence, whereas calcium pyrophosphate would be positively birefringent?


Ben 04.25.15 Reply

You are correct. The question is being purposefully coy, making sure you know that the classic location (first MTP) and high-ish uric acid go with bonafide gout.

djdlbh 02.24.16 Reply

Hi Ben ,
I cannot thank you ( and the person who put your link up on sdn ) enough for all the effort you put in here .

I WISH all NBMEs were available like this too .

I took a practice test recently at my prometric centre but they used the old 2013 version of questions for me .I cannot find that set of questions online anymore since the usmle updated its content in 2015 .Would you happen to have a link to that ?

Also I have a few doubts regarding some questions but I dont see much activity here after april 2015 , wonder if you are still around to help answer some questions regarding this thread .Thanks

Ben 02.24.16 Reply

Here is a link to the archived pdf. Archive.org is a lifesaver sometimes.

djdlbh 02.24.16 Reply

Q51 B – Crossed findings (upper motor neuron on one side, lower motor neuron on the other) means a brainstem lesion. Left (ipsilateral) tongue, right-sided (contralateral) weakness means the exiting left hypoglossal nerve has been affected (within the left medulla). B is in the pyramid where the corticospinal tract runs to control muscles (prior to the decussation).

label E is the location of the hypoglossal nerve .How can we choose B over E ? I kept looking at this question for a good minute and then guessed randomly between B and E because in my opinion both are correct . Chose E and moved on .Did I read the Q wrong ? was it only asking about the pyramidal tract ?

Ben 02.24.16 Reply

This is known as medial medullary syndrome (or Dejerine syndrome occasionally inferior alternating hemiplegia). This wiki page may be helpful.

A lesion in E would would be a hypoglossal nucleus injury and would explain the tongue findings but not the hemiplegia. Only a lesion in the pyramids at this level can explain a LMN tongue lesion (exiting CN 12 fibers) and the associated hemiparesis in this context. The corticospinal tract and its fibers supplying the arm and leg are in the pyramids of the dorsal medulla. Thus a lesion in the ventral medulla (E) wouldn’t cause hemiparesis.

josh 02.26.16 Reply

Cranial nerve nuclei are LMNs .That was my understanding .E is hypoglossal nerve nucleus right ?

Ben 02.27.16 Reply

It is. However, a hypoglossal nucleus lesion would not cause contralateral hemiplegia.

josh 02.26.16 Reply

Wikipedia says that supranuclear nerve lesions are crossed .
“Testing function of the nerve is performed by asking the subject to stick their tongue straight out. If there is a loss of innervation to one side, the tongue will curve toward the affected side, due to unopposed action of the opposite genioglossus muscle. If this is the result of a lower motor neuron lesion, the tongue will be curved toward the damaged side, combined with the presence of fasciculations or atrophy. However, if the deficit is caused by an upper motor neuron lesion, the tongue will be curved away from the side of the cortical damage”

Mike Raj 01.02.17 Reply

Hi Ben,

I am really impressed with your incredible help for offering explanation for old USMLE Part 1 questions for the last 3 years. I need a small help. Would you happen to have any explanations for released exams from 2006 to 2013? If so, can you please send me the explanation files to my email id? This will help me a lot to prepare. Thank you so much


Ben 01.03.17 Reply

I don’t sorry; I only started doing these back in 2013. Though Kaplan at some point used to do explanations several years ago, I don’t know how many years they did, and I don’t believe any explanations for the earlier years are available anywhere anymore, if they ever existed.

Con 01.27.17 Reply

Hi Ben, thanks for all your explanations, they’re really helpful. I have a quick question on Block 3’s 113. I’m wondering why you can say definitively that it’s 1/2 since I was under the impression that Fragile X is an X-Linked Dominant disorder. As such, I get that you’d get 2 of the 4 girls (statistically) to carry the defective X from the mother, but would they still be labeled “carriers” in this case? Taking into account X-inactivation, would the values change at all?

Ben 01.28.17 Reply

Yes, they’re still carriers. The carrier state is determined by the germline, not the phenotype. When female carriers make eggs, each egg has one of their two X-chromosomes in it. X-inactivation plays no role in gametogenesis, because by definition each egg only gets one copy (and has equal likelihood of containing the faulty or good copy).

X-inactivation does play a role in phenotype. So carriers of X-linked diseases like Duchenne muscular dystrophy or fragile X can have varying degrees of the mild condition/disease. A companion question of relevance here is #23 on the 2016 set.

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