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NBME CBSSA practice questions

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Question 1USMLE Step 1 preparation

A 24-year-old man sustains a midshaft humerus fracture. Two weeks later he is unable to extend the wrist and has decreased sensation over the dorsum of the hand between the thumb and index finger. Which nerve is most likely injured?

  1. Radial nerve Correct answer.
  2. Ulnar nerve
  3. Musculocutaneous nerve
  4. Median nerve

Explanation

The radial nerve runs in the spiral groove against the midshaft of the humerus, so fractures at this level commonly injure it. Injury produces wrist drop and sensory loss over the first dorsal web space. The other nerves listed are not positioned to be injured by a midshaft humeral fracture.

Question 2USMLE Step 1 preparation

During the cardiac cycle, which event coincides with the first heart sound?

  1. Closure of the atrioventricular valves Correct answer.
  2. Rapid ventricular filling
  3. Opening of the aortic valve
  4. Closure of the semilunar valves

Explanation

The first heart sound is produced by closure of the mitral and tricuspid valves at the onset of ventricular systole. Semilunar valve closure generates the second heart sound, and valve opening is normally silent. Rapid filling occurs in early diastole, well before S1.

Question 3USMLE Step 1 preparation

A 7-year-old child develops a sore throat with fever and tonsillar exudates. Two weeks later he presents with cola-colored urine, periorbital edema, and hypertension. Which finding on urinalysis is most characteristic of this condition?

  1. White blood cell casts
  2. Oval fat bodies
  3. Muddy brown casts
  4. Red blood cell casts Correct answer.

Explanation

This is poststreptococcal glomerulonephritis, an immune complex-mediated glomerular disease presenting with nephritic syndrome. Red blood cell casts indicate glomerular bleeding and are the hallmark urinary finding. The other casts reflect tubular or interstitial processes rather than glomerular inflammation.

Question 4USMLE Step 1 preparation

Which vitamin deficiency is most directly associated with megaloblastic anemia and elevated serum homocysteine?

  1. Vitamin C
  2. Vitamin K
  3. Vitamin B12 Correct answer.
  4. Vitamin D

Explanation

Vitamin B12 is a cofactor for methionine synthase, which converts homocysteine to methionine; deficiency raises homocysteine and impairs DNA synthesis, causing megaloblastic anemia. Vitamin C, K, and D deficiencies do not produce megaloblastic changes.

Question 5USMLE Step 1 preparation

A 30-year-old woman has heat intolerance, weight loss, tremor, and diffuse goiter. Laboratory tests show low TSH and elevated free T4. Which autoantibody most specifically supports Graves disease?

  1. Rheumatoid factor
  2. Antinuclear antibodies
  3. Antibodies against the TSH receptor Correct answer.
  4. Antibodies against thyroid peroxidase alone

Explanation

Graves disease is caused by TSH receptor-stimulating antibodies that drive diffuse hyperthyroidism and goiter. These antibodies are the most specific serologic marker for Graves disease. Antinuclear antibodies and rheumatoid factor are markers of other autoimmune conditions.

Question 6USMLE Step 1 preparation

A 55-year-old man with chronic alcohol use presents with confusion, ataxia, and ophthalmoplegia. Which structure is most classically affected in this condition?

  1. Substantia nigra
  2. Caudate nucleus
  3. Cerebellar vermis
  4. Mammillary bodies Correct answer.

Explanation

The triad of confusion, ataxia, and ophthalmoplegia indicates Wernicke encephalopathy, caused by thiamine deficiency. It classically produces hemorrhagic necrosis of the mammillary bodies and periaqueductal gray. The substantia nigra and caudate are affected in other basal ganglia disorders.

Question 7USMLE Step 1 preparation

A 2-year-old child has recurrent sinopulmonary infections and is found to have absent B cells with very low immunoglobulins. T cell numbers and function are normal. Which diagnosis is most likely?

  1. X-linked agammaglobulinemia Correct answer.
  2. Chronic granulomatous disease
  3. Severe combined immunodeficiency
  4. DiGeorge syndrome

Explanation

X-linked agammaglobulinemia results from a BTK mutation that blocks B cell maturation, causing absent B cells and hypogammaglobulinemia with intact T cell function. It presents after maternal IgG wanes with recurrent bacterial sinopulmonary infections. The other disorders affect T cells or phagocytes.

Question 8USMLE Step 1 preparation

Which enzyme is the rate-limiting step of glycolysis?

  1. Pyruvate kinase
  2. Aldolase
  3. Hexokinase
  4. Phosphofructokinase-1 Correct answer.

Explanation

Phosphofructokinase-1 catalyzes the committed, rate-limiting step of glycolysis and is regulated by ATP, citrate, AMP, and fructose-2,6-bisphosphate. Hexokinase and pyruvate kinase are also regulated but are not the rate-limiting step.

Question 9USMLE Step 1 preparation

A 68-year-old man has a resting tremor, cogwheel rigidity, and bradykinesia. Which pathologic finding is most characteristic of his disease?

  1. Demyelination of the optic nerve
  2. Spongiform change in the cortex
  3. Lewy bodies in the substantia nigra Correct answer.
  4. Neurofibrillary tangles in the hippocampus

Explanation

Parkinson disease results from loss of dopaminergic neurons in the substantia nigra with eosinophilic Lewy bodies containing alpha-synuclein. The clinical picture described is the classic triad of parkinsonism. The other findings belong to different neurodegenerative diseases.

Question 10USMLE Step 1 preparation

A 45-year-old woman has a blood pressure of 170/105 mm Hg and hypokalemia. Plasma aldosterone is elevated and plasma renin is suppressed. Which is the most likely cause?

  1. Heart failure with reduced ejection fraction
  2. Renal artery stenosis
  3. Renin-secreting tumor
  4. Aldosterone-producing adrenal adenoma Correct answer.

Explanation

Primary hyperaldosteronism from an adrenal adenoma causes autonomous aldosterone secretion that suppresses renin and drives hypertension with hypokalemia. Secondary hyperaldosteronism from renal hypoperfusion would show elevated renin. This distinction is the key diagnostic step.

Question 11USMLE Step 1 preparation

A 60-year-old man with long-standing hypertension develops flank pain and hematuria. Imaging shows an irregular mass in the renal cortex. Which renal tumor is most likely?

  1. Transitional cell carcinoma
  2. Clear cell renal cell carcinoma Correct answer.
  3. Renal oncocytoma
  4. Wilms tumor

Explanation

Clear cell renal cell carcinoma is the most common primary renal malignancy in adults and arises from proximal tubule cells, often presenting with hematuria, flank pain, and a cortical mass. Transitional cell carcinoma arises in the renal pelvis and presents with painless hematuria.

Question 12USMLE Step 1 preparation

A 32-year-old woman has a butterfly rash, photosensitivity, and arthralgias. Which autoantibody is most specific for her diagnosis?

  1. Anti-double-stranded DNA Correct answer.
  2. Anti-centromere
  3. Anti-Scl-70
  4. Anti-histone

Explanation

Systemic lupus erythematosus is most specifically marked by antibodies against double-stranded DNA, which also correlate with lupus nephritis. Anti-histone antibodies are associated with drug-induced lupus and are less specific. Anti-centromere and anti-Scl-70 belong to scleroderma variants.

Question 13USMLE Step 1 preparation

A newborn is noted to have a continuous machinery murmur at the left infraclavicular area with bounding pulses. Which mechanism explains the murmur?

  1. Right-to-left shunting across an atrial septal defect
  2. Regurgitation across the tricuspid valve
  3. Obstruction of the aortic valve
  4. Left-to-right shunting through a patent ductus arteriosus Correct answer.

Explanation

A patent ductus arteriosus creates continuous flow from the higher-pressure aorta into the pulmonary artery, producing a machinery murmur and widened pulse pressure. Right-to-left shunts cause cyanosis rather than this murmur. Aortic stenosis and tricuspid regurgitation produce different murmurs and no bounding pulses.

Question 14USMLE Step 1 preparation

A 70-year-old woman has bone pain and fatigue, with hypercalcemia, renal insufficiency, and anemia. Serum protein electrophoresis shows a monoclonal spike. Which complication is most directly caused by the monoclonal light chains?

  1. Osteolytic bone lesions
  2. Recurrent bacterial pneumonia
  3. Renal tubular damage Correct answer.
  4. Hyperviscosity from intact immunoglobulins

Explanation

In multiple myeloma, filtered light chains (Bence Jones proteins) are directly toxic to renal tubules and cause cast nephropathy. Bone lesions result from osteoclast activation by cytokines, and infection results from impaired humoral immunity. Hyperviscosity relates to the intact paraprotein.

Question 15USMLE Step 1 preparation

A 50-year-old man has jaundice, right upper quadrant pain, and weight loss. Imaging shows a mass in the pancreatic head with dilation of the common bile duct and pancreatic duct. Which tumor marker is most likely elevated?

  1. Carcinoembryonic antigen
  2. CA 19-9 Correct answer.
  3. Alpha-fetoprotein
  4. Beta-hCG

Explanation

Pancreatic head adenocarcinoma obstructs both the common bile duct and the pancreatic duct, producing the double-duct sign, and is associated with elevated CA 19-9. Alpha-fetoprotein marks hepatocellular carcinoma and germ cell tumors, not pancreatic cancer.

Question 16USMLE Step 1 preparation

A patient with chronic kidney disease is given a drug that is cleared primarily by the kidney. Which pharmacokinetic parameter is most likely to increase if the dose is unchanged?

  1. Hepatic first-pass metabolism
  2. Area under the plasma concentration-time curve Correct answer.
  3. Bioavailability
  4. Volume of distribution

Explanation

Reduced renal clearance causes drug accumulation, which increases total systemic exposure as reflected by the area under the concentration-time curve. Volume of distribution and bioavailability are not directly altered by impaired renal elimination. Hepatic first-pass metabolism is unrelated to renal clearance.

Question 17USMLE Step 1 preparation

A 19-year-old college student has fever, sore throat, lymphadenopathy, and atypical lymphocytes on peripheral smear. A heterophile antibody test is positive. Which virus is responsible?

  1. Herpes simplex virus type 1
  2. Epstein-Barr virus Correct answer.
  3. Cytomegalovirus
  4. Human herpesvirus 6

Explanation

Infectious mononucleosis with a positive heterophile antibody test is caused by Epstein-Barr virus, which infects B cells via CD21. Cytomegalovirus can cause a heterophile-negative mononucleosis-like illness. The other herpesviruses cause different clinical syndromes.

Question 18USMLE Step 1 preparation

A 65-year-old man has progressive dyspnea and cough. Biopsy of a lung mass shows small blue cells with scant cytoplasm and no gland formation. Which paraneoplastic syndrome is most characteristic?

  1. Syndrome of inappropriate antidiuretic hormone secretion Correct answer.
  2. Carcinoid syndrome
  3. Hypercalcemia from parathyroid hormone-related peptide
  4. Hypoglycemia from insulinoma

Explanation

Small cell lung carcinoma is a neuroendocrine tumor that commonly secretes ADH ectopically, causing SIADH with hyponatremia. Paraneoplastic hypercalcemia via PTHrP is more typical of squamous cell carcinoma. Carcinoid syndrome and insulinoma are separate neuroendocrine entities.

Question 19USMLE Step 1 preparation

A 28-year-old man has acute severe abdominal pain, and imaging shows free air under the diaphragm. Which type of necrosis is most likely occurring in the affected tissue?

  1. Fibrinoid necrosis
  2. Fat necrosis
  3. Caseous necrosis
  4. Coagulative necrosis Correct answer.

Explanation

Ischemic injury from a perforated viscus produces coagulative necrosis, in which tissue architecture is preserved and cells become eosinophilic and anucleate. Caseous necrosis occurs in tuberculosis, fat necrosis in pancreatitis, and fibrinoid necrosis in vasculitis.

Question 20USMLE Step 1 preparation

A 45-year-old woman has episodic headache, palpitations, and diaphoresis with severe hypertension. Plasma free metanephrines are elevated. Which tumor is most likely?

  1. Parathyroid adenoma
  2. Adrenocortical adenoma
  3. Pituitary prolactinoma
  4. Pheochromocytoma Correct answer.

Explanation

Pheochromocytoma is a catecholamine-secreting tumor of the adrenal medulla that classically produces episodic headache, palpitations, diaphoresis, and hypertension. Elevated plasma free metanephrines are the preferred screening test. Adrenocortical and pituitary tumors do not secrete catecholamines.

Question 21USMLE Step 1 preparation

A 3-year-old child has fever, petechiae, and bruising. Laboratory tests show anemia, thrombocytopenia, and many lymphoblasts with blasts exceeding 20% of marrow cells. Which finding is most characteristic of the most likely diagnosis?

  1. Philadelphia chromosome
  2. Terminal deoxynucleotidyl transferase positivity Correct answer.
  3. Auer rods
  4. Smudge cells

Explanation

Acute lymphoblastic leukemia is the most common childhood leukemia and is characterized by lymphoblasts that are positive for terminal deoxynucleotidyl transferase. Auer rods indicate acute myeloid leukemia, and smudge cells suggest chronic lymphocytic leukemia.

Question 22USMLE Step 1 preparation

A 62-year-old man has a chronic cough and weight loss. Sputum cytology shows keratinized cells with intercellular bridges. Which paraneoplastic electrolyte abnormality is most likely?

  1. Hypoglycemia
  2. Hypercalcemia Correct answer.
  3. Hyperkalemia
  4. Hyponatremia

Explanation

Keratin pearls and intercellular bridges identify squamous cell carcinoma of the lung, which can secrete parathyroid hormone-related peptide and cause humoral hypercalcemia of malignancy. Hyponatremia from SIADH is typical of small cell carcinoma instead.

Question 23USMLE Step 1 preparation

A 25-year-old woman has a malar rash and is treated with a drug that inhibits dihydrofolate reductase. Which metabolite level will rise as a marker of impaired folate metabolism?

  1. Ceruloplasmin
  2. Methylmalonic acid
  3. Homocysteine Correct answer.
  4. Ferritin

Explanation

Methotrexate inhibits dihydrofolate reductase and reduces tetrahydrofolate, impairing remethylation of homocysteine to methionine, so homocysteine rises. Methylmalonic acid rises in vitamin B12 deficiency because the methylmalonyl-CoA mutase reaction is impaired, not in folate deficiency.

Question 24USMLE Step 1 preparation

A 55-year-old man with cirrhosis develops confusion, asterixis, and elevated ammonia. Which mechanism best explains the neurologic findings?

  1. Cerebral vasospasm from subarachnoid blood
  2. Impaired hepatic clearance of ammonia and false neurotransmitter accumulation Correct answer.
  3. Acute demyelination of peripheral nerves
  4. Excess dopamine in the basal ganglia

Explanation

Hepatic encephalopathy results when portosystemic shunting and hepatocellular failure reduce ammonia clearance, promoting astrocyte swelling and accumulation of false neurotransmitters such as octopamine. The other mechanisms describe unrelated neurologic disorders.

Question 25USMLE Step 1 preparation

A 40-year-old woman has episodic palpitations and anxiety. Her plasma glucose is low during an attack and symptoms resolve with eating. Which finding would best localize the source?

  1. Elevated C-peptide with elevated insulin Correct answer.
  2. Suppressed C-peptide with elevated insulin
  3. Elevated cortisol with low ACTH
  4. Elevated catecholamines with normal glucose

Explanation

Endogenous hyperinsulinism from an insulinoma shows high insulin with high C-peptide because the tumor secretes proinsulin-derived peptides. Exogenous insulin administration would show high insulin with suppressed C-peptide. The other patterns do not explain fasting hypoglycemia.

Question 26USMLE Step 1 preparation

A 24-year-old man sustains a penetrating injury to the posterior triangle of the neck. On examination he cannot elevate his right shoulder, and the scapula is displaced laterally and inferiorly with winging. Which nerve has most likely been injured?

  1. Long thoracic nerve
  2. Spinal accessory nerve Correct answer.
  3. Suprascapular nerve
  4. Axillary nerve

Explanation

The spinal accessory nerve (CN XI) supplies the trapezius, which elevates and retracts the scapula; injury in the posterior triangle produces shoulder droop, weakness of shoulder elevation, and lateral displacement of the scapula. The long thoracic nerve supplies serratus anterior and causes true medial winging, while the axillary and suprascapular nerves serve the deltoid and supraspinatus/infraspinatus respectively.

Question 27USMLE Step 1 preparation

A 30-year-old woman has recurrent kidney stones and is found to have a low serum phosphate with an inappropriately normal serum calcium. Which hormone is most likely elevated?

  1. Calcitonin
  2. 1,25-dihydroxyvitamin D
  3. Parathyroid hormone
  4. Fibroblast growth factor 23 Correct answer.

Explanation

Fibroblast growth factor 23 is produced by osteocytes and acts on the proximal tubule to cause phosphaturia while suppressing 1-alpha-hydroxylase, so it lowers both phosphate and 1,25-dihydroxyvitamin D. Elevated FGF23 with normocalcemia fits tumor-induced osteomalacia or an inherited hypophosphatemic disorder.

Question 28USMLE Step 1 preparation

During the cardiac cycle, which event coincides most closely with the first heart sound?

  1. Closure of the aortic and pulmonic valves
  2. Rapid ventricular filling
  3. Opening of the mitral valve
  4. Closure of the atrioventricular valves Correct answer.

Explanation

The first heart sound is produced by closure of the mitral and tricuspid valves at the onset of ventricular systole, when ventricular pressure exceeds atrial pressure. Semilunar valve closure generates the second sound, and valve opening and rapid filling are normally silent.

Question 29USMLE Step 1 preparation

A 58-year-old man with a history of peptic ulcer disease develops severe epigastric pain radiating to the back. Serum lipase is markedly elevated and abdominal CT shows pancreatic necrosis. Which mechanism best explains the hypocalcemia that may accompany this illness?

  1. Impaired hepatic synthesis of vitamin D-binding protein
  2. Increased secretion of calcitonin from pancreatic islets
  3. Saponification of peripancreatic fat by released lipases Correct answer.
  4. Renal tubular loss of calcium induced by pancreatitis

Explanation

In acute pancreatitis, released pancreatic lipases digest peripancreatic fat into fatty acids that bind calcium in a process called saponification, sequestering calcium and lowering the serum level. Fat necrosis with chalky deposits is the pathologic correlate of this sequestration.

Question 30USMLE Step 1 preparation

A 9-month-old infant presents with a harsh systolic murmur along the left sternal border and is found to have a ventricular septal defect. Which hemodynamic consequence is most likely present?

  1. Complete mixing of systemic and pulmonary venous blood
  2. Left-to-right shunt with increased pulmonary blood flow Correct answer.
  3. Right-to-left shunt with cyanosis at birth
  4. Obstruction to left ventricular outflow

Explanation

Because left ventricular pressure exceeds right ventricular pressure after birth, an uncomplicated ventricular septal defect produces a left-to-right shunt that increases pulmonary blood flow and may cause pulmonary congestion and failure to thrive. Cyanosis would indicate reversal of the shunt from pulmonary hypertension (Eisenmenger physiology).

Question 31USMLE Step 1 preparation

A 22-year-old man is found to have a deficiency of the enzyme that converts phenylalanine to tyrosine. Which of the following is the most direct consequence of this defect?

  1. Accumulation of phenylalanine with deficient tyrosine Correct answer.
  2. Accumulation of homogentisic acid in cartilage
  3. Impaired conversion of tyrosine to thyroxine
  4. Excess production of dopamine from phenylalanine

Explanation

Phenylalanine hydroxylase catalyzes the conversion of phenylalanine to tyrosine using tetrahydrobiopterin as a cofactor. Its deficiency causes phenylketonuria, with elevated blood phenylalanine and inadequate tyrosine, which can produce fair skin and lighter hair.

Question 32USMLE Step 1 preparation

A 47-year-old woman with type 2 diabetes is started on a drug that inhibits the sodium-glucose cotransporter 2 in the proximal tubule. Which of the following best describes the expected effect of this agent?

  1. Stimulation of insulin secretion from beta cells
  2. Reduced hepatic gluconeogenesis
  3. Increased renal glucose reabsorption
  4. Glucosuria with osmotic diuresis Correct answer.

Explanation

SGLT2 reabsorbs most filtered glucose in the early proximal tubule; inhibiting it causes glucosuria, which lowers plasma glucose and produces an osmotic diuresis. This mechanism is insulin-independent, so it does not rely on beta-cell function.

Question 33USMLE Step 1 preparation

A 35-year-old woman has a history of recurrent fetal loss and a prolonged activated partial thromboplastin time that does not correct with mixing of normal plasma. Which antibody is most likely responsible?

  1. Antibody against factor VIII
  2. Antibody against beta-2 glycoprotein I Correct answer.
  3. Antibody against ADAMTS13
  4. Antibody against platelet glycoprotein IIb/IIIa

Explanation

Antiphospholipid syndrome is characterized by antibodies against phospholipid-binding proteins such as beta-2 glycoprotein I and cardiolipin, producing a paradoxical prolonged aPTT that fails to correct on mixing because the inhibitor targets the phospholipid reagent. Clinically it causes arterial and venous thrombosis and recurrent pregnancy loss.

Question 34USMLE Step 1 preparation

A 66-year-old man with a 40-pack-year smoking history has hemoptysis and a central lung mass. Biopsy shows nests of cells with keratin pearls and intercellular bridges. Which of the following is most likely to be elevated in his serum?

  1. Calcitonin
  2. Alpha-fetoprotein
  3. Carcinoembryonic antigen
  4. Parathyroid hormone-related peptide Correct answer.

Explanation

Squamous cell carcinoma of the lung is a central, keratinizing tumor associated with smoking, and it classically secretes parathyroid hormone-related peptide, causing hypercalcemia of malignancy. Alpha-fetoprotein, CEA, and calcitonin are tumor markers for other malignancies.

Question 35USMLE Step 1 preparation

A 28-year-old woman presents with heat intolerance, weight loss, and a diffuse goiter. Radioactive iodine uptake is elevated. Which mechanism best explains the increased thyroid hormone synthesis in Graves disease?

  1. Autonomous nodular secretion of hormone
  2. Excess pituitary secretion of TSH
  3. Destruction of thyroid follicles by cytotoxic T cells
  4. Thyroid-stimulating immunoglobulins activating the TSH receptor Correct answer.

Explanation

Graves disease is caused by thyroid-stimulating immunoglobulins that bind and activate the TSH receptor, driving diffuse goiter and hormone synthesis with elevated radioactive iodine uptake. This contrasts with the low uptake seen in destructive thyroiditis.

Question 36USMLE Step 1 preparation

A 6-year-old boy is brought in after a dog bite. He has never been vaccinated against rabies. Which of the following best describes the appropriate post-exposure management?

  1. Observation only, because animal bites rarely transmit rabies
  2. Immediate administration of rabies immune globulin and rabies vaccine Correct answer.
  3. Administration of rabies immune globulin alone, without vaccine
  4. Administration of rabies vaccine alone, without immune globulin

Explanation

For an unvaccinated person with a significant exposure, post-exposure prophylaxis requires both passive immunity with rabies immune globulin, infiltrated around the wound, and active immunization with the rabies vaccine series. Immune globulin is omitted in patients who were previously vaccinated.

Question 37USMLE Step 1 preparation

A 52-year-old man has chronic kidney disease with a creatinine clearance of 20 mL/min. A drug that is primarily eliminated unchanged by the kidney is prescribed at the usual dose. Which pharmacokinetic parameter will most likely be increased?

  1. Volume of distribution
  2. Elimination half-life Correct answer.
  3. First-pass metabolism
  4. Bioavailability

Explanation

Elimination half-life is proportional to volume of distribution and inversely proportional to clearance; when renal clearance falls, total clearance decreases and half-life lengthens, so the drug accumulates at usual dosing. Volume of distribution is largely independent of renal function.

Question 38USMLE Step 1 preparation

A 60-year-old man with a long history of poorly controlled hypertension has a lacunar infarct in the internal capsule. Which of the following is the most important underlying vascular change?

  1. Rupture of a berry aneurysm
  2. Embolic occlusion from a left atrial thrombus
  3. Lipohyalinosis of small penetrating arteries Correct answer.
  4. Atherosclerotic occlusion of the middle cerebral artery

Explanation

Lacunar infarcts result from disease of small penetrating arteries, typically lipohyalinosis and hyaline arteriolosclerosis caused by chronic hypertension, producing small deep infarcts in the basal ganglia, thalamus, and internal capsule. Large-vessel atherosclerosis and emboli produce cortical infarcts.

Question 39USMLE Step 1 preparation

A 40-year-old woman with bipolar disorder treated with lithium presents with coarse tremor, ataxia, and confusion. Lithium therapy is discontinued, but she develops polyuria and dilute urine. Which renal tubular defect best explains this complication?

  1. Impaired bicarbonate reabsorption in the proximal tubule
  2. Impaired glucose reabsorption in the proximal tubule
  3. Impaired vasopressin-mediated water reabsorption in the collecting duct Correct answer.
  4. Impaired sodium reabsorption in the thick ascending limb

Explanation

Lithium accumulates in principal cells of the collecting duct and interferes with vasopressin signaling through aquaporin-2, producing nephrogenic diabetes insipidus with polyuria and dilute urine that does not respond to desmopressin. Loop and thiazide diuretics act at other nephron segments.

Question 40USMLE Step 1 preparation

A 3-year-old boy is brought in with bruising and gum bleeding. His platelet count and coagulation studies are normal, but the bleeding time is prolonged and platelet aggregation fails with ristocetin and is corrected by adding normal plasma. Which diagnosis is most likely?

  1. Bernard-Soulier syndrome
  2. Hemophilia A
  3. Glanzmann thrombasthenia
  4. von Willebrand disease Correct answer.

Explanation

von Willebrand disease causes defective platelet adhesion because von Willebrand factor is deficient or dysfunctional; ristocetin-induced aggregation is abnormal and corrects with normal plasma, which supplies the missing factor. Platelet count and coagulation times are typically normal in mild disease.

Question 41USMLE Step 2 CK preparation

A 68-year-old man with a 40-pack-year smoking history presents with 3 weeks of progressive dyspnea and a new cough. Chest radiograph shows a large right-sided pleural effusion, and thoracentesis yields bloody fluid. Which of the following is the most appropriate next step in management?

  1. Send pleural fluid for cytology and obtain chest tube drainage Correct answer.
  2. Place the patient in Trendelenburg position
  3. Reassure and discharge with outpatient follow-up
  4. Begin empiric antituberculous therapy

Explanation

A bloody exudative effusion in an older smoker raises concern for malignancy, so fluid should be sent for cytology and the effusion drained to relieve dyspnea. Empiric TB therapy is not indicated without supporting findings, and discharge is unsafe given the size of the effusion.

Question 42USMLE Step 2 CK preparation

A 24-year-old woman at 34 weeks gestation presents with a 2-day history of right upper quadrant pain, nausea, and blood pressure of 158/104 mmHg. Laboratory studies show platelets 88,000/µL, AST 180 U/L, and creatinine 1.3 mg/dL. Which of the following is the most likely diagnosis?

  1. Acute cholecystitis
  2. Acute fatty liver of pregnancy
  3. Viral hepatitis
  4. HELLP syndrome Correct answer.

Explanation

Hemolysis, elevated liver enzymes, and low platelets after 20 weeks gestation with hypertension define HELLP syndrome, a severe variant of preeclampsia. The combination of thrombocytopenia, transaminitis, and elevated creatinine supports this diagnosis rather than isolated hepatobiliary disease.

Question 43USMLE Step 2 CK preparation

A 45-year-old man presents with 6 months of episodic palpitations, diaphoresis, and headache. Blood pressure is 190/110 mmHg during an episode. Which of the following is the most appropriate initial screening test?

  1. Renal artery Doppler ultrasonography
  2. Serum cortisol at midnight
  3. Serum thyrotropin
  4. Plasma free metanephrines Correct answer.

Explanation

Episodic headache, sweating, and palpitations with paroxysmal hypertension suggest pheochromocytoma, for which plasma free metanephrines are the most sensitive initial test. Thyroid studies and cortisol testing evaluate different causes of hypertension and would not detect a catecholamine-secreting tumor.

Question 44USMLE Step 2 CK preparation

A 3-year-old child is brought in with fever to 39.5°C, drooling, and refusal to swallow. He is sitting upright and leaning forward, and there is no cough. Which of the following is the most appropriate next step?

  1. Inspect the oropharynx with a tongue blade
  2. Give oral amoxicillin and discharge
  3. Obtain a lateral neck radiograph in the emergency department
  4. Perform immediate endotracheal intubation in the operating room Correct answer.

Explanation

High fever, drooling, refusal to swallow, and tripod positioning suggest epiglottitis with a threatened airway. The priority is controlled airway management in the operating room by experienced pediatric anesthesia/ENT personnel with rescue-airway capability. Imaging or forceful throat examination should not delay airway protection.

Question 45USMLE Step 2 CK preparation

A 58-year-old woman reports 4 months of fatigue, weight gain, cold intolerance, and constipation. Examination shows dry skin and delayed relaxation of the deep tendon reflexes. Which of the following laboratory findings is most consistent with the diagnosis?

  1. High TSH, high free T4
  2. Low TSH, low free T4
  3. Low TSH, high free T4
  4. High TSH, low free T4 Correct answer.

Explanation

Primary hypothyroidism produces an elevated TSH from loss of negative feedback along with a low free T4. The clinical picture of cold intolerance, weight gain, and delayed reflex relaxation is classic for hypothyroidism.

Question 46USMLE Step 2 CK preparation

A 30-year-old woman presents with 2 days of dysuria, frequency, and suprapubic discomfort. She is afebrile with no flank pain. Urinalysis shows pyuria and nitrite positivity. Which of the following is the most appropriate treatment?

  1. Ciprofloxacin for 14 days
  2. Observation without antibiotics
  3. Intravenous ceftriaxone
  4. Nitrofurantoin for 5 days Correct answer.

Explanation

Uncomplicated cystitis in a woman is treated with a short course of an oral agent such as nitrofurantoin for 5 days. Fluoroquinolones are reserved for more complicated infections because of collateral damage and resistance concerns.

Question 47USMLE Step 2 CK preparation

A 72-year-old man is found to have a blood pressure of 168/94 mmHg on two separate visits. He has no symptoms and no evidence of end-organ damage. Which of the following is the most appropriate initial management?

  1. Admit for intravenous antihypertensive therapy
  2. Start lifestyle modification and a thiazide diuretic Correct answer.
  3. Start a beta-blocker and an ACE inhibitor simultaneously
  4. Recheck blood pressure in 1 year

Explanation

Asymptomatic stage 2 hypertension without end-organ damage is managed with lifestyle modification plus a single oral agent, commonly a thiazide diuretic, with reassessment in about a month. Intravenous therapy is reserved for hypertensive emergency with end-organ injury.

Question 48USMLE Step 2 CK preparation

A 26-year-old woman at 10 weeks gestation presents with vaginal bleeding and cramping. The cervix is closed and no tissue is passed. Ultrasonography shows an intrauterine gestational sac with no fetal cardiac activity and a crown-rump length of 12 mm. Which of the following is the most likely diagnosis?

  1. Missed abortion Correct answer.
  2. Incomplete abortion
  3. Threatened abortion
  4. Ectopic pregnancy

Explanation

A nonviable intrauterine pregnancy with a closed cervix and no passage of tissue is a missed abortion. Threatened abortion would show fetal cardiac activity, and incomplete abortion would involve passage of tissue with an open cervix.

Question 49USMLE Step 2 CK preparation

A 62-year-old man with cirrhosis presents with confusion and asterixis. He has not had a bowel movement in 4 days. Which of the following is the most appropriate initial pharmacologic therapy?

  1. Lactulose Correct answer.
  2. Metronidazole
  3. Haloperidol
  4. Rifaximin alone

Explanation

Hepatic encephalopathy is initially treated by reducing ammonia production with lactulose titrated to two to three soft stools daily. Rifaximin is typically added for secondary prophylaxis after a recurrent episode rather than used alone for acute management.

Question 50USMLE Step 2 CK preparation

A 55-year-old woman presents with sudden severe headache described as the worst of her life. Noncontrast head CT is normal, but she remains symptomatic 6 hours after onset. Which of the following is the most appropriate next step?

  1. Observation and repeat CT in 24 hours
  2. Start sumatriptan
  3. MRI of the brain without contrast
  4. Lumbar puncture for xanthochromia Correct answer.

Explanation

A thunderclap headache with a normal CT still requires lumbar puncture to assess for xanthochromia, which detects subarachnoid hemorrhage when CT is negative early. Sumatriptan treats migraine and would delay diagnosis of a potentially fatal hemorrhage.