Free USMLE Step 1 Exam Questions and Answers

A 31-year-old woman comes to the physician because of increased thirst and urination for 3 weeks. She has bipolar disorder, and her mood has been stable since lithium therapy was started 2 years ago. She drinks about 5 L of water daily. Her temperature is 36.8°C (98.2°F), pulse is 76/min, and blood pressure is 118/74 mm Hg. Laboratory studies show: Serum Na+ 146 mEq/L Glucose 92 mg/dL Lithium 0.8 mEq/L (therapeutic range, 0.6-1.2) Urine osmolality 142 mOsm/kg H2O Urine osmolality does not change after administration of desmopressin. Which of the following is the most likely mechanism of this patient's polyuria?

• AA. Osmotic diuresis from impaired glucose reabsorption
• BB. Inhibition of Na+-K+-2Cl− cotransport in the thick ascending limb
• CC. Decreased ADH synthesis in the supraoptic nucleus
• DD. Impaired aquaporin-2 insertion in collecting duct cells
• EE. Primary stimulation of hypothalamic thirst centers

A 29-year-old woman comes to the physician because of blurred vision and pain with movement of her left eye for 4 days. Eight months ago she had a transient episode of numbness and tingling in her right leg that resolved without treatment. Examination shows decreased visual acuity and a relative afferent pupillary defect in the left eye. Sensation to light touch is decreased over the right lower extremity. MRI of the brain shows multiple periventricular white matter lesions oriented perpendicular to the ventricles. Cerebrospinal fluid analysis shows oligoclonal IgG bands. Immune-mediated destruction of which of the following cell types is most likely responsible for this patient's condition?

• AA. Schwann cells
• BB. Astrocytes
• CC. Ependymal cells
• DD. Microglia
• EE. Oligodendrocytes

A 25-year-old man is brought to the emergency department 10 minutes after being stung on the neck by a bee while hiking. He reports difficulty breathing and lightheadedness. He has no history of medical problems. His pulse is 128/min, respirations are 28/min, and blood pressure is 76/40 mm Hg. Examination shows diffuse urticaria, lip swelling, and wheezing over both lung fields. Which of the following is the most appropriate next step in management?

• AA. Inhaled albuterol
• BB. Intravenous norepinephrine
• CC. Intravenous diphenhydramine
• DD. Intravenous methylprednisolone
• EE. Intramuscular epinephrine

A 4-year-old boy is brought to the physician because of frequent falls and difficulty climbing stairs. He began walking at 20 months of age. When rising from the floor, he places his hands on his thighs and pushes himself upright. Examination shows enlarged, firm calves and weakness of the hip girdle muscles. Serum creatine kinase activity is 18,000 U/L (N=22–198). His maternal uncle had a similar illness and used a wheelchair by age 12 years. A muscle biopsy specimen shows variation in fiber size, necrosis, and fibrofatty replacement; immunostaining for dystrophin is completely absent. Which of the following genetic changes is most likely in this patient?

• AA. Point mutation in the gene encoding the alpha subunit of the acetylcholine receptor
• BB. In-frame deletion of exons encoding the central rod domain of the protein
• CC. Frameshift deletion that introduces a premature stop codon in the gene
• DD. Missense substitution of glycine in the triple-helical region of a collagen gene
• EE. Expansion of a CTG repeat in the 3' untranslated region of the gene

A 22-year-old African American man comes to the emergency department because of dark urine, fatigue, and low back pain for 1 day. Three days ago he started trimethoprim-sulfamethoxazole for a urinary tract infection. He has had no previous similar episodes. His temperature is 37.2°C (99°F), pulse is 104/min, and blood pressure is 118/72 mm Hg. Examination shows scleral icterus. Laboratory studies show: Hemoglobin 8.2 g/dL Reticulocyte count 6% Bilirubin, total 3.4 mg/dL Bilirubin, direct 0.4 mg/dL Lactate dehydrogenase 640 U/L A peripheral blood smear after supravital staining shows red blood cells with inclusions, and some cells have semicircular defects. Impaired production of which of the following in red blood cells is most likely responsible for this patient's condition?

• AA. NADPH
• BB. ATP
• CC. Pyruvate
• DD. FADH2
• EE. NADH

A 19-year-old man with type 1 diabetes mellitus is brought to the emergency department because of 2 days of vomiting, abdominal pain, and increasing lethargy. He ran out of insulin 4 days ago. His respirations are deep and rapid at 30/min. Arterial blood gas analysis and serum studies show: pH 7.26 Na+ 136 mEq/L Cl− 98 mEq/L HCO3− 10 mEq/L Glucose 520 mg/dL Assuming appropriate respiratory compensation, which of the following is the expected arterial PCO2 in this patient?

• AA. 18 mm Hg
• BB. 23 mm Hg
• CC. 30 mm Hg
• DD. 40 mm Hg
• EE. 52 mm Hg

Researchers investigate a possible association between regular use of a certain herbicide and pancreatic cancer. They identify 120 patients with newly diagnosed pancreatic cancer and 240 age- and sex-matched patients without cancer from the same hospitals. All participants are interviewed about their past occupational exposure to the herbicide. Which of the following is the most appropriate measure of association for this study?

• AA. Odds ratio
• BB. Standardized mortality ratio
• CC. Relative risk
• DD. Hazard ratio
• EE. Attributable risk

A 70-year-old man with chronic systolic heart failure is admitted because of 3 days of worsening dyspnea, orthopnea, and leg swelling. Examination shows jugular venous distention, bibasilar crackles, and 3+ pitting edema to the knees. He is given intravenous furosemide and has a brisk increase in urine output within 30 minutes. This drug most likely acts by inhibiting which of the following?

• AA. Na+-Cl− cotransporter in the early distal convoluted tubule
• BB. Carbonic anhydrase in the proximal convoluted tubule
• CC. Epithelial Na+ channel in the cortical collecting duct
• DD. Aquaporin-2 channel in the principal cells of the collecting duct
• EE. Na+-K+-2Cl− cotransporter in the thick ascending limb

A 58-year-old man is hospitalized for evaluation of weight loss and jaundice. A CT scan of the abdomen shows a pancreatic mass with liver metastases, and biopsy results confirm adenocarcinoma. The patient is alert, oriented, and has decision-making capacity. He has not named anyone to receive information on his behalf and has not previously told the physician how much he wants to know. As the physician approaches his room, the patient's daughter stops her in the hallway and says, "Please don't tell my father it's cancer. He will give up if he finds out." Which of the following is the most appropriate response by the physician?

• AA. "I will share the results with you first so that you can decide when and how to tell him."
• BB. "He is entitled to know the results, so I will give him the full diagnosis and prognosis today."
• CC. "The ethics committee should decide whether he is told, so I will refer the matter to them."
• DD. "I understand you want to protect him, and I will first ask him how much he wants to know."
• EE. "I will tell him only what he asks about, and I will not bring up the prognosis, as you have requested."

A 58-year-old man comes to the physician because of a 6-month history of progressive difficulty swallowing both solids and liquids. He regurgitates undigested food, has a nocturnal cough, and has had a 5-kg (11-lb) weight loss. He does not smoke or drink alcohol. Vital signs are within normal limits. Physical examination shows no abnormalities. A barium swallow shows a dilated esophagus that tapers distally to a narrow segment. Esophageal manometry shows incomplete relaxation of the lower esophageal sphincter with swallowing and absent peristalsis in the esophageal body. Endoscopy shows no mass or stricture. Which of the following is the most likely underlying cause of this patient's condition?

• AA. Outpouching of mucosa through a muscular defect in the pharyngeal wall
• BB. Fibrosis of the esophageal submucosa with smooth muscle atrophy
• CC. Degeneration of inhibitory neurons in the myenteric plexus
• DD. Herniation of the gastric cardia through the diaphragmatic hiatus
• EE. Metaplasia of squamous epithelium to columnar epithelium at the gastroesophageal junction