Explanations for the 2018 Official Step 1 Practice Questions

Here are my explanations for the new NBME 2018 USMLE Step 1 Sample Test Questions. This year there are 51 new ones (marked with asterisks).

Like in years past, the question order here is for the PDF version (not the FRED-simulated browser version). This facilitates using these explanations in future years when they change the available question set (because the old ones are always available via archive.org). The multimedia explanations are at the end.

Prior sets/explanations can be found here.

Block 1

1. D – Schistosomiasis is a parasitic worm particularly endemic in Africa (Egypt, in particular, comes up the most on questions) that is most associated with chronic cystitis. Calcifications of the bladder wall are essentially pathognomonic. Chronic infection is associated with an increased risk of squamous cell carcinoma of the bladder (as opposed to the usual urothelial/transitional cell).
2. A – Basic nerve anatomy and function.*
3. D – Hot tub folliculitis, it’s a thing. Classically pseudomonas.*
4. A – Acting out aka “being a teenager.”*
5. E – VEGF is a major tissue growth factor activated by injury, cytokine release (infection, inflammation) and hypoxia that promotes angiogenesis and also increases vascular permeability (hence the edema). This increased permeability aids in the movement of proteins and white blood cells to the site of injury.
6. C – Pseudogout is caused by positively-birefringent rhomboid calcium pyrophosphate crystal deposition. Most commonly affected location is the knee. Contrast with gout, negatively birefringent, big toe. A differential for both to keep in mind is septic arthritis.*
7. C – Filgrastim is a granulocyte colony stimulating factor (GCSF), which are drugs used to increase white blood cell count in patients with leukopenia. Leucovorin (folinic acid) sounds like it would also be right, but it’s used to prevent bone marrow suppression in patients taking methotrexate. Darbepoetin (like erythropoietin) is used to stimulate red blood cell production.
8. B –  The left-sided system is much higher pressure than the right side, hence the aortic valve closing is usually louder than the pulmonic valve. A P2 louder than A2 means that the pulmonary arterial pressure is significantly elevated.
9. C – Southern blots are commonly used in immunological studies, as the southern blot allows for the study of DNA alterations. What is normally one gene configuration related to immune globulins in most tissues demonstrates multiple different bands in the bone marrow, indicative of gene rearrangement. This is basically how we create new antibodies. Reactive processes are polyclonal (multiple bands); leukemia, in contrast, is monoclonal (single band).*
10. F – Total peripheral resistance goes down during exercise as the arterioles supplying muscle and skin dilate.*
11. D – The baroreceptors are stretch receptors (the more fluid in the vessel, the more they fire). So a patient with hemorrhagic shock will see a decrease in the baroreceptor firing rate. Activation of RAAS will result in increased vascular resistance (vasoconstriction) in order to maintain blood pressure. And capillaries, such as those in the kidney, will be primed for resorption and not filtration (no one wants to pee out good dilute urine when they’re dehydrated). Likewise, systemic capillaries will prefer to hold onto plasma and not let it leak into the interstitium (third-spacing).
12. E – PCP is a sedative-hypnotic and dissociative anesthetic that generally acts as a downer but can also cause incredible aggression coupled with pain insensitivity (the superman drug). Vertical nystagmus is a commonly mentioned physical exam finding.
13. C – The Pouch of Douglas is the space between the uterus and the rectum (i.e. the place where pelvic free fluid goes).
14. D – The “migratory serpiginous perianal rash” (ick) is classic for strongyloides, a parasitic roundworm acquired from larvae-contaminated soil. Strongyloides larvae can borrow (hence the rash) and can migrate to the GI tract and lay their eggs, which then hatch in the intestine and cause diarrhea. Treatment is Ivermectin (and if not, mebendazole/albendazole). Checking the stools for larvae is the most sensitive test. Parasite life cycles are gross.
15. C – Leydig cells make testosterone. Leydig cell tumors aren’t always physiologically active, but those that are can cause masculinization. Granulosa cell tumors, on the other hand, sometimes produce estrogen (which can lead to precocious puberty in young girls but otherwise may be occult). Teratomas are oddballs that typically have fat, hair, teeth, etc. Thecomas will not be on your test. Ovarian carcinoid is highly unlikely to show up on your test, but if it did, it would likely present with a classic carcinoid syndrome.
16. A – Pulmonary fibrosis (a restrictive pattern disease) is a major cause of mortality for patients with scleroderma. Logically, if the disease causes fibrosis elsewhere, it’s going to cause fibrosis in the lungs.
17. D – Sensitivity rules things out. It’s TP / (TP + FN). So in order to calculate the sensitivity of this test, we need the true positives (the 90 with cancer) and the false negatives: the patients for whom the test is negative but actually do have prostate cancer. That’s D.
18. B – Bisphosphonates work by decreasing osteoclast activity (thereby reducing bone resorption). Choice F is the opposite of how estrogen therapy works (RANKL is found on osteoblasts, and its activation triggers osteoclasts and stimulates bone resorption).
19. B – Memorize aspirin’s unique acid-base effects: metabolic acidosis and respiratory alkalosis. Note, this is likely actual respiratory alkalosis, not simply normal respiratory compensation for metabolic acidosis.
20. B – Note that the question is not asking what cells fight URIs. The question asks what lab finding would be consistent with decreased immune activity (and thus the only choice that matches “decreased” with an immune cell is the best answer).
21. B – Don’t let them blind you with this patient’s misery. The issue of the day is that he has a DVT. That’s why he came to the ER in the first sentence and what the ultrasound shows at the end. Patients with cancer are hypercoagulable.*
22. E – Endothelial tight junctions’ permeability is increased in response to injury and inflammation, allowing migration of white blood cells and friends to the site of injury.
23. A – Endemic Burkitt lymphoma can happen in Brazil as well as Africa (jaw lesion, puffy face). The photomicrograph is demonstrating tingible body macrophages, a type of macrophage containing many phagocytized, apoptotic cells in various states of degradation.
24. A – p53 is the quintessential tumor suppressor (it activates apoptosis). HPV carcinogenesis is caused by insertion of the virus into host DNA that produces a protein which binds to an essential p53 substrate, functionally inactivating p53 and preventing its apoptotic cascade. C (transactivation/TAX) is how HIV and HTLV cause cancer. E (c-myc translocation) causes Burkitt lymphoma.
25. A – DI is an important complication of some skull base fractures and can be treated with DDAVP. You probably remember that this works via the activation of aquaporin channels, but these are moved from intracellular vesicles to the apical membrane surface as a result of a DDAVP-mediated increase adenylate cyclase via a stimulatory G protein that increases intracellular cAMP.*
26. B – Crohn’s: skip lesions, fistulae, strictures (and the unnecessary transmural involvement on histology).
27. C – The mom will pass on her deletion in 50%. The father will pass it on in 100% (because both of his copies are affected). Therefore, the child will automatically have at least one deletion and will have the double deletion in 50%.*
28. C – Approximate fasting physiology timing: the post-absorptive phase (6-24 hours after a meal) is dominated by glycogenolysis. Gluconeogenesis from 24 hours to 2 days. Then ketosis.*
29. B – The arrow is pointing to a neutrophil (multilobed nucleus): main fighter of the immune system in acute inflammation and bacterial infection (such as aspiration pneumonia). C5a is a chemotactic factor for PMNs.
30. C – While E coli is normal gut flora, your body would prefer it stay intraluminal.*
31. A – Air and fluid = hydropneumothorax. If that fluid is blood (s/p stabbing), it’s a hemopneumothorax. Lack of mediastinal shift indicates that it’s not under tension.
32. E – von Willebrand disease is by far the most common inherited bleeding diathesis. Frequently, the only laboratory abnormality is increased bleeding time (literally you prick the patient and see how long it takes them to stop bleeding). On Step, bleeding women have VWD. Bleeding boys have hemophilia.
33. E – Fragile X is a CGG trinucleotide repeat expansion disorder (which like Huntington’s is a test favorite). The maternal uncle is the hint to the X-linked inheritance. Autism-like behaviors and relatively large head are common; large testicles only appear after puberty.
34. E – Gram-positive rods in a diabetic foot wound (or a World War I soldier fighting in a trench) means Clostridium perfringens (the causative organism of gas gangrene). Crepitus means gas in the tissues, which is produced as a byproduct of its highly virulent alpha toxin.
35. D – He has (presumably RSV) bronchiolitis. RSV is an RNA virus that enters the cell via a fusion protein (which is the target of the prophylactic monoclonal antibody drug Palivizumab).*
36. D – A b2 agonist like the bronchodilator albuterol would sure help that wheezing. Note that epinephrine (such as in an epi-pen) would also achieve this but is nonselective; in this case, the patient’s symptoms would be helped most by the beta-2 component.*
37. C – The purpose of Rhogam is to bind to and remove the RhD antigens so that the mother does not form an immune response against the antigen in fetus’ blood. It’s given to at-risk Rh- moms at 28 weeks and at delivery.*
38. C – Metformin is awesome. It decreases hepatic glucose production, decreases intestinal absorption of glucose, and improves insulin sensitivity by increasing peripheral glucose uptake and utilization.*
39. A – Gonorrhea can change its pilus, which is responsible for adhesion to host cells and the main antigen to which the host mounts an immune response. Neisseria gonorrhoeae is able to switch out different pilin genes, and for this reason, prior infection does not confer long-lasting immunity.
40. B – Anorexia leads to hypogonadotropic hypogonadism, as the body realizes that the possibility of nourishing a fetus is zero and gives up the pretense. There’s a lot of supporting data, but one should guess this answer once you read the word “gymnast” (or “dancer”).

Block 2

41. C – Statins raise HDL and decrease LDL and TGs. Their effect on LDL is by far the most potent, but they do a little good on everything.
42. E – Androgens stimulate sebaceous glands and cause acne. In girls, this is primarily due to adrenarche (DHEA/DHEAS androgen production made by the adrenal gland the zona reticularis). Boys can also blame testosterone from gonadal puberty (pubarche).
43. D –Diffuse low-level ST elevation means pericarditis. These patients often complain of pleuritic chest pain that is somewhat alleviated by sitting up and leaning forward and have distant heart sounds. Common test causes include viruses, uremia, and 2-3 weeks after myocardial infarction (Dressler syndrome).
44. A – Electrical alternans on boards means a big pericardial effusion (and usually cardiac tamponade physiology). The heart cannot fill properly, preload decreases, hypotension and tachycardia ensue, fluid backup leads to elevated JVP. Underlying etiology in this patient is renal failure.
45. B – Codeine is a prodrug with basically no analgesic effects by itself. It instead must be metabolized (mostly by the liver via CYP2D6) into morphine in order to provide analgesia. Some folks convert more, some less.*
46. A – Ah, countertransference. Remember that transference is when the patient is transferring (redirecting) feelings about someone on to you (you remind them of their dad). Countertransference is when you do it about them (they remind you of your son). Projection is when you assign your own feelings to them (you are angry, so you think you they are angry).
47. C – Serum sickness! A type III (immune complex) hypersensitivity.
48. B – You will remember that G6PD deficiency causes red blood cells to break down in response to certain stressors, infections, and drugs. The patient’s symptoms are a manifestation of indirect hyperbilirubinemia due to hemolysis (RBC breakdown). Sulfa, fava beans, nitrofurantoin, isoniazid, and antimalarials (e.g. primaquine) are the common test favorites.* This question was also updated since the original PDF was released, as a helpful reader pointed out below. It is a now a case of bubonic plague (or possibly ulceroglandular Tularemia), both which are treated by answer choice E, which refers to aminoglycosides, which target the 30S ribosomal subunit. Please see the 2019 explanations.
49. D – The effects of excess thyroid hormone: attempted compensatory TSH suppression, increase in both T4 and free T4, and normal TBG. Note that the question doesn’t even hinge on TBG and is also unlikely to on the real thing.
50. D – Carbamazepine is a notorious CYP450 inducer, so you should be guessing metabolism no matter what. CYP450 plays an important role in both vitamin D bioactivation and degradation in the liver.*
51. C – This is obviously a clinical trial. If you know you are getting a drug, then you are not blinded: it’s an open-label trial. There is no randomization as there is only a single treatment group.
52. G –Vincristine (a mitosis inhibitor) frequently causes peripheral neuropathy, which can be severe and irreversible. Other fun associations are Bleomycin with pulmonary fibrosis, Cyclophosphamide and bladder cancer, and Doxorubicin with dilated cardiomyopathy.
53. C – Acetylcholine increases after drug X, which is the same we’d expect if drug X were a cholinesterase inhibitor.*
54. B – This question is a little bit BS, in that there is nothing in the stem at all to make you think this specifically. What the question is trying to ask is what factors cause malpractice suits in general. While professional competence is a cause, people love to discuss and the boards love to test about poor physician communication and lack of empathy as root causes.
55. D – The scenario described is heart failure 2/2 mitral valve regurgitation. If we want to prove the regurg, we can confirm and grade it using an echo.*
56. E – Pubertal gynecomastia in males is normal and generally goes away on its own. If “normal” is an answer choice, make really sure you don’t want to pick it.
57. E – Multiple infections. Abscesses. Then you hear decreased oxidative burst and immediately think NADPH oxidase deficiency aka Chronic granulomatous disease, which causes recurrent abscess-forming infections due to the inability to kill ingested organisms because of the inability to generate superoxide radicals.*
58. D – Relationship with former patients are generally frowned on, but they’re especially problematic if the patient was a psychiatric patient, as the power imbalance of the practitioner-patient relationship and information the provider is privy to because of their patient care involvement preclude a healthy balanced relationship of equals.
59. E – Subacute combined degeneration (progressive peripheral sensory and motor loss) is a late sign of B12 deficiency, which is common in old people. On 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.
60. A – The infraspinatus and teres minor are responsible for external rotation. Both the infraspinatus and supraspinatus muscles are innervated by a suprascapular nerve.
61. A – Fear = amygdala*
62. A – The alpha-value corresponds to the p-value we will accept as significant and reflects the likelihood of a type I error (a false positive). A lower alpha-value means a lower acceptable likelihood of obtaining the same results by chance, and thus, significant results can be reported more confidently (a 1% false positive rate instead of a 5% rate).
63. C – Swallowing amniotic fluid is a critical component of lung development. Fetuses with severe oligohydramnios are plagued by pulmonary hypoplasia, which is the cause of death in fetuses born with Potter syndrome (renal agenesis).
64. C – This question is asking for the vascular supply of the parathyroid glands. That would be the inferior thyroid arteries, which arise from the thyrocervical trunk.*
65. B – Thiazides (typically used as antihypertensives) also increase calcium resorption in the distal tubule and are therefore useful in preventing calcium oxalate stone formation in patients with hypercalciuria (the mechanism is not really worth learning). Thiazides block the Na-Cl symporter, as opposed to loop diuretics, which block the triporter, and acetazolamide, which blocks carbonic anhydrase in the proximal tubule.
66. A – A new blistering disease in an older person is typically going to be a pemphigus question. Then you just have to remember the difference between bullous pemphigoid vs pemphigus vulgaris. Bullous pemphigoid is characterized by the loss of hemidesmosomes that bind keratinocytes to the basement membrane, resulting in bulla (big blisters) in areas of friction, choice A. Patients with pemphigus vulgaris lose their desmosomes (which bind keratinocytes to each other), so that their skin is super friable, which results in ulceration. Mouth ulcers are more common in PV.
67. B – The Lincoln’s beard distribution described is that of the V3 branch of the trigeminal nerve, which exits the skull base via foramen ovale. V2 exits via foramen rotundum. V1 exits via the superior orbital fissure.*
68. E – Splitting is an immature defense mechanism often employed by patients with borderline personality disorder. When splitting, a person fails to see others as capable of having both positive and negative qualities; at any given time, it’s all or nothing.
69. D – PMA activates NADPH-oxidase, such as seen in the oxidative burst. In this case, this stimulation produces no response, suggesting NADPH-oxidase deficiency (which means this is another question about chronic granulomatous disease). The organism should thus be an abscess forming bacteria such as staph aureus. Oh wait, that’s what the question described anyway (two-year-old with recurrent abscesses), meaning that the PMA nonsense was basically superfluous.* (If your answer key says E, then you’re looking at the 2019 set. See the explanation for the newer version of this question here).
70. A – As always, it’s almost better to ignore the pictures when possible. This gentleman has a peptic ulcer, which we know is caused predominately by H. pylori infection. H. pylori produces proteases and particularly urease, which allow it to increase the pH of its local environment by cleaving urea into ammonia, which is toxic to gastric mucosa. The picture demonstrates H pylori, which are evident with silver staining.
71. B – A genetic variation in a particular nucleotide is by definition a polymorphism. Note that the question specifically states that it does not change the protein.*
72. C – TTP has a classic pentad: microangiopathic hemolytic anemia, thrombotic purpura, fever, renal failure, neurologic abnormalities (AMS). Whenever you see a question where the patient suddenly has a lot going on, consider TTP.*
73. A – Androgen insensitivity is caused by a defective androgen receptor. DHT is responsible for creating male genitalia during fetal sexual development. The default human gender is female. So a genetically male patient with complete androgen insensitivity is externally phenotypically female. Lack of response to adrenal androgens prevents hair formation during puberty (adrenarche).
74. A – Intermittent hyperbilirubinemia/jaundice in an otherwise healthy individual is typical of Gilbert’s syndrome, which is caused by the decreased activity of UDP glucuronosyltransferase.*
75. A – An annular pancreas surrounds the duodenum and can cause intermittent duodenal obstruction. While this question theoretically requires the imaging to answer correctly, the only other choice that is feasible is D, which is known as SMA syndrome. SMA syndrome is quite rare and typically seen in people who have recently had significant weight loss. On the imaging, it would be smooshing of the duodenum by a bright contrast filled artery as opposed to surrounding by soft tissue. I also think it’s highly unlikely to be tested.*
76. A – The usual cold-like symptoms of runny eyes and a sore throat are common of several strains of adenovirus that are readily communicated amongst humans in close contact.*
77. D – The arrowed fluid is contained in a space behind the stomach but in front of the retroperitoneal structures (e.g. the pancreas), i.e. the lesser sac.
78. B – This is a (prospective) case series. There is no control group (and certainly no blinding).
79. E – Calcium oxalate stones are the most common variety of kidney stones, but uric acid stones make up 5-10% as well. None of the other choices are associated with renal calculi of any variety.
80. E – The patient’s chronic inflammatory pneumonitis is killing off his lung parenchyma (composed primarily of type I pneumocytes). Type II pneumocytes, in addition to making surfactant, can replicate in order to replace type I pneumocytes, so they will be increased. Chronic interstitial inflammation results in fibrosis, hence an increase in fibroblasts.

Block 3

81. A – Malonyl-CoA inhibits the rate-limiting step in the beta-oxidation of fatty acid. Logically, resting muscle requires less energy (and thus less need for fatty acid breakdown) than active muscle.
82. E – Timolol is a beta-blocker. Beta blockers can exacerbate asthma/COPD/reactive airway disease.*
83. E – No relation between the atrium (P) waves and the QRS complex means third-degree AV block (aka “complete” heart block). Symptomatic (even fatal) bradycardia can result. “Cannon” a(trial) waves are prominent jugular venous pulsations that occur when the atria and ventricle contract simultaneously (which, of course, doesn’t normally happen).*
84. E – CNS amoebiasis is most notoriously caused by Naegleria fowleri, which I encourage you to memorize as the “brain-eating amoeba.” Found in fresh-water bodies of water like ponds and lakes, it has three forms: a cyst, a trophozoite (ameboid), and a biflagellate (i.e. has two flagella). Infection is via olfactory cell axons through the cribriform plate to the brain.*
85. E – Functional parathyroid adenomas can cause elevated parathyroid hormone (PTH), which results in hypercalcemia and hypophosphatemia. Hypercalcemia is characterized by the rhyming symptoms: stones (renal, biliary), bones (including bone pain to osteitis fibrosa cystica), groans (abdominal pain, n/v), thrones (polyuria, constipation), and psychiatric overtones (from depression to coma).
86. A – The main downside of live vaccines is that they rarely cause the disease they’re designed to prevent, typically in immune-compromised individuals (who either get the vaccine or are close contact to someone who does)*.
87. B – They have described what you assume is a classic case of pneumonia. But, PNA isn’t an answer choice. What the next best thing? The cause! Old frail people (and alcoholics) love to get aspiration pneumonia. RLL is the most common site, which they have provided (thank you, big vertical bronchus). They even gave you the hint that the patient has “difficulty swallowing,” which is code for “aspirates when swallowing.”*
88. B – The proliferative phase of the menstrual cycle is controlled by cyclin-dependent kinases.*
89. C – Stroke characterized by left hemiparesis and right CN12 palsy. Crossed findings mean a brainstem lesion. Right (ipsilateral) tongue, left-sided (contralateral) weakness means the exiting right hypoglossal nerve has been affected (within the right medulla). C is the pyramid where the corticospinal tract runs to control muscles (prior to the decussation). This is known as the medial medullary syndrome or Dejerine syndrome.
90. E – This patient has chronic kidney disease, as indicated by elevated serum creatinine/BUN and evidence of anemia of chronic disease (normochromic normocytic). Poorly functioning kidneys do not hydroxylate 25-Dihydroxycholecalciferol to 1,25-Dihydroxycholecalciferol well nor produce adequate erythropoietin (hence the CKD-related anemia). Patient’s with CKD thus develop secondary hyperparathyroidism due to deranged phosphate excretion and inadequate Vitamin D activation resulting in hypocalcemia.  Thus, we should expect to see low calcium, high phosphorus, low 1,25 vitamin D, and low Epo, which is E.
91. C – There is an inferior orbital wall blow-out fracture. If you happen to know CT anatomy, this is actually involving the infraorbital foramen, which transmits the infraorbital artery and nerve, but really, so long as you notice the obvious fracture it’s the only choice that makes sense.*
92. C – Stress-related bowel issues, sometimes but not always alternatively involving both diarrhea and constipation, are a hallmark of irritable bowel syndrome (IBS). A diagnosis of exclusion, most of the stem is helping you rule out more serious issues. Lubiprostone, which is a fatty acid that induces stool-softening intestinal secretions, is approved for idiopathic constipation, most commonly in the context of IBS and or secondary to opiate use. The other drugs listed are for IBD.*
93. B – Blood at the meatus is the red flag (see what I did there?) for urethral injury, which should be evaluated for with a retrograde urethrogram. The membranous the most commonly injured by fracture. In contrast, the spongy urethra is most likely to be injured during traumatic catheter insertion or in a straddle injury.*
94. A – Oral vesicle (hint hint). Blistering vesicular lesion on the hand. No fever, not toxic-appearing. This is Herpes (you may remember dentists getting herpetic whitlow in your studies, which is what this is). Most folks get HSV1 as children, though obviously not all are symptomatic. HSV is a large double-stranded, linear DNA virus.*
95. F – Many autoimmune/autoinflammatory conditions, including ankylosing spondylitis, are treated with DMARDs like anti-TNF-alpha medications when milder stuff doesn’t do the trick. Common examples are infliximab (Remicade) and adalimumab (Humira).*
96. D – If the fluid keeps coming into the glomerulus (via the afferent arteriole), but you clamp the exiting vessel (the efferent arteriole), then it’s going to build up in the glomerulus, leading to increased hydrostatic pressure.*
97. D – SIADH is a test favorite a very common cause of hyponatremia (after dehydration). A variety of brain and lung pathologies are possible etiologies, with lung cancer (of any type) being an important cause.
98. E – DMD is X-linked. We know her mom is a carrier based on family history, supported by lab testing. But her mom has 2 X chromosomes, only one of which is mutated. There is no way to know which her daughter eventually receives and expresses by her phenotype (i.e if she is a carrier or not). Just because her CK is normal doesn’t mean she isn’t a carrier–the phenotype of the X-linked carrier depends on X-inactivation.
99. B – Absolute risk reduction is the decrease in the number affected per number exposed = (15-5)/50 = 10/50 = 0.2.*
100. B – This requires no explanation.*
101. B – What we have here is a congenital intolerance to breast milk: galactosemia, in which the body cannot convert galactose to glucose (resulting in an accumulation of Galactose 1-phosphate). They then list the findings and tests used to diagnose it. Lactose (the disaccharide in milk) is composed of glucose + galactose.
102. C – Just because he’s having (unprotected) sex doesn’t mean he doesn’t have simple infectious mononucleosis. The sex implies he’s also kissing someone! Pharyngitis + lymph nodes + fatigue = mono.
103. D – An odds ratio greater than 1 signifies increases odds/risk/likelihood. If the 95% confidence interval range does not include 1, then the difference is statistically significant (though not necessarily clinically meaningful).
104. C – Osgood-Schlatter is also known as apophysitis of the tibial tubercle. It’s due to chronic stress/irritation at the insertion of the patellar tendon on the tibial tubercle. It’s classically seen in teenagers doing repetitive vigorous activity (running, jumping). The radiograph demonstrates classic fragmentation of the tibial tubercle (which isn’t necessary to recognize to get the question correct).
105. B – BK virus reactivation is a cause of renal damage post-transplant in the setting of immune suppression, also known as BK nephropathy. Treatment is a reduction in immune suppression, allowing the body a chance to fight back. The virus can cause a cold-like URI syndrome with fever.*
106. E – Specificity is the ability to accurately predict true negatives: TN / (TN + FP) = 95 / (95+5) = 95%*
107. E – Cystic fibrosis is an autosomal recessive disease involving CFTR, which means you need a double hit to express the disease. If the genetic test only picked up one, then it must have missed the other.*
108. B – (Unstable) angina. Most immediate treatment is nitro.*
109. B – To amplify tiny fragments of DNA in order to detect their presence, we use PCR. The question is a description of the process. Southern Blots are used to detect a specific DNA sequence within a DNA sample.
110. A – Targetoid rash after a woodland excursion means Lyme disease, caused by Borrelia burgdorferi, carried by the Ixodes tick. Rash (erythema migrans), viral syndrome symptoms, fatigue, and polyarthritis are common. Lyme carditis typically manifests as AV block.
111. A – Narcotic use for acutely painful conditions is both reasonable and important. (Very) short-term use (immediately post-surgical) does not lead to long-term dependence (or so people have thought…). And yes, drugs addicts should also receive narcotics to control pain.
112. D – Many oral cavity lesions, especially anteriorly such as the tip of the tongue, drain first to submental nodes (level 1). Oropharyngeal SCCs most commonly drain to level 2.*
113. C – This patient has hepatitis (elevated liver enzymes) due to active Hepatitis C infection. Hep C and HIV infection are both associated with intravenous drug use. While most patients with Hep A will clear the virus after their acute illness, Hep C causes chronic infection in 80% of patients, which may lead to cirrhosis over time (~20 years).
114. B – p53 is an important tumor suppressor gene, particularly in its ability to cause a cell to undergo apoptosis in the event of damage. p53 activity also holds the cell at the G1/S regulation point (B), limiting DNA synthesis.
115. D – The only thing that directly raises BP of the list is increased PVR.*
116. D – Below the dentate line, anal cancer drainage is superficial inguinal. Above the dentate line, superior rectal (then iliac).*
117. C – A history of volume loss (often GI 2/2 vomiting, diarrhea) resulting in shock is consistent with a hypovolemic etiology, as corroborated by the plethora of physical evidence provided. Diuretics exacerbate the situation, working against your body’s desire to retain fluid to compensate.

Multimedia

38. E – Classic Moro reflex, entirely expected and normal until it disappears around age 4 months
39. E – I’m going to point out that a normal healthy kid with no cardiac history or symptoms and no family history of sudden cardiac death for a pre-sports physical is probably going to have a benign exam no matter what you think you hear.
40. E – Pill-rolling resting tremor of Parkinson’s disease secondary to loss of dopamine neurons in the substantia nigra.

Requests for further clarifications etc can be made in the comments below. Your best bet for score correlations is probably here.

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
— How I read NBME/USMLE Questions
— Free USMLE Step 1 Questions