Free USMLE Step 1 Exam Questions and Answers

A healthy girl is brought to the clinic for a well-child examination. She sits steadily without support and passes a block from one hand to the other. She babbles "mama" and "dada" but does not use the words for her parents specifically. She cries and clings to her mother when a visiting relative tries to hold her. She pulls to stand while holding furniture but cannot walk on her own. Her growth parameters are at the 50th percentile. Which of the following is the most likely age of this child?

• AA. 4 months
• BB. 24 months
• CC. 2 months
• DD. 15 months
• EE. 9 months

An 81-year-old woman comes for a routine examination. She lives independently and takes no medications. Her serum creatinine concentration is 0.9 mg/dL, the same as it was at age 50. Which of the following best describes the expected age-related change in her physiology?

• AA. Decreased pulmonary residual volume
• BB. Increased vital capacity
• CC. Decreased glomerular filtration rate, masked by reduced creatinine production
• DD. Increased lean body mass relative to fat mass
• EE. Increased proportion of slow-wave (stage N3) sleep

A 24-year-old man is treated with trimethoprim-sulfamethoxazole for a urinary tract infection. Three days later he has dark urine, fatigue and scleral icterus. Hemoglobin is 9.1 g/dL, the reticulocyte count is 7%, serum LDH is raised and haptoglobin is low. A peripheral smear shows red cells with semicircular "bites" removed. A supravital stain shows small dark inclusions inside the red cells. The underlying disorder most directly impairs which of the following processes in his erythrocytes?

• AA. Interaction between spectrin and ankyrin in the membrane skeleton
• BB. ATP generation by pyruvate kinase
• CC. Synthesis of heme in the mitochondria
• DD. Anchoring of complement-regulatory proteins to the cell membrane
• EE. Regeneration of reduced glutathione

A 63-year-old woman with long-standing rheumatoid arthritis has fatigue. Hemoglobin is 10.2 g/dL and the MCV is 81 μm³. Serum iron is low, total iron-binding capacity is low, and serum ferritin is increased. Which of the following is the most likely cause of her low serum iron?

• AA. Reduced erythropoietin receptor expression on erythroid precursors
• BB. Decreased synthesis of the β-globin chain
• CC. Increased hepatic secretion of hepcidin
• DD. Inhibition of ferrochelatase by lead
• EE. Chronic occult gastrointestinal blood loss

A 16-year-old girl has had heavy menstrual bleeding since menarche and frequent nosebleeds. Her mother bruises easily. Platelet count is 260,000/mm³ and PT is normal. PTT is mildly prolonged and bleeding time is prolonged. Ristocetin-induced platelet agglutination is decreased. The deficient protein most directly mediates which of the following?

• AA. Aggregation of platelets through fibrinogen binding to glycoprotein IIb/IIIa
• BB. Adhesion of platelets to exposed subendothelial collagen through glycoprotein Ib
• CC. Cleavage of large multimers of von Willebrand factor
• DD. γ-Carboxylation of glutamate residues on clotting factors
• EE. Inactivation of factors Va and VIIIa

A 34-year-old woman has 1 week of fatigue, gum bleeding and bruising. Laboratory studies show pancytopenia, a prolonged PT and PTT, low fibrinogen and raised D-dimer. A bone marrow aspirate shows numerous abnormal promyelocytes, many containing bundles of rod-shaped cytoplasmic inclusions. Cytogenetics confirms the expected translocation. The most appropriate initial targeted pharmacotherapy acts by which of the following mechanisms?

• AA. Inhibiting DNA methyltransferase
• BB. Binding the retinoic acid receptor-α portion of a fusion protein, which allows the blasts to differentiate
• CC. Inhibiting Bruton tyrosine kinase
• DD. Inhibiting the BCR-ABL tyrosine kinase
• EE. Depleting CD20-expressing B cells

A 10-month-old boy has had three episodes of otitis media and two of pneumonia since age 6 months. Cultures grew *Streptococcus pneumoniae* and *Haemophilus influenzae*. His tonsils are barely visible. Serum IgG, IgA and IgM are all markedly low. Flow cytometry shows almost no CD19⁺ cells, and CD3⁺ cell counts are normal. A maternal uncle died of infection in childhood. Which of the following is most likely defective?

• AA. Adenosine deaminase
• BB. Common γ chain of the interleukin-2 receptor
• CC. Bruton tyrosine kinase
• DD. NADPH oxidase
• EE. CD40 ligand on T cells

A 38-year-old man with a history of recurrent sinus infections and two episodes of giardiasis receives a unit of packed red blood cells after major trauma. Within 5 minutes he develops generalised urticaria, wheezing and a blood pressure of 72/40 mm Hg. He has no fever, and studies show no evidence of hemolysis. Which of the following measures would most likely have prevented this reaction?

• AA. Repeat ABO typing and crossmatch
• BB. Premedication with acetaminophen
• CC. Use of washed red cells or products from IgA-deficient donors
• DD. Leukoreduction of the blood product
• EE. Gamma irradiation of the blood product

A 41-year-old man receives polyvalent antivenom of animal origin after a rattlesnake bite. Nine days later he develops fever, an itchy urticarial rash, pain in multiple joints and mild proteinuria. Serum C3 and C4 are low. Which of the following is the most likely mechanism of his symptoms?

• AA. Antibody binding to fixed antigens on cell surfaces
• BB. CD8⁺ T-cell–mediated killing of host cells
• CC. Cross-linking of IgE on mast cells
• DD. Th1-mediated macrophage activation
• EE. Deposition of antigen–antibody complexes in vessel walls with complement activation

A 66-year-old man with hypertension suddenly develops vertigo, hoarseness and difficulty swallowing. Examination shows the following: - Left-sided ptosis and miosis. - Loss of pain and temperature sensation on the **left** side of the face and the **right** side of the body. - Left-sided limb ataxia. - Strength is normal throughout. Occlusion of which of the following arteries is most likely?

• AA. Left posterior inferior cerebellar artery
• BB. Anterior inferior cerebellar artery
• CC. Right posterior cerebral artery
• DD. Anterior spinal artery
• EE. Paramedian branches of the basilar artery