USMLE Step 1 · USMLE Step 1: Medical Sciences

Biochemistry and Genetics

32 practice questions, each with the answer and an explanation.

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Random questions from this topic, one at a time. You see the answer and the explanation straight after each one.

All 32 questions

  1. A child has megaloblastic anemia and orotic aciduria without hyperammonemia. Which supplement treats this?
  2. A 20-year-old has recurrent pancreatitis, pulmonary infections and infertility. Which gene is defective?
  3. A 50-year-old alcoholic has confusion, ataxia and ophthalmoplegia. Which vitamin is deficient?
  4. A 25-year-old has recurrent sinus infections and infertility. Chest imaging shows dextrocardia. Which structure is defective?
  5. A child has hypotonia, a flat facial profile and a single palmar crease. What is the most common cause of this syndrome?
  6. A sailor has bleeding gums, perifollicular hemorrhages and poor wound healing. Which process is impaired?
  7. A patient has diarrhea, dermatitis and dementia. Which vitamin is deficient?
  8. A boy has self-mutilating behavior, gout and intellectual disability. Which enzyme is deficient?
  9. A boy has hematuria, sensorineural hearing loss and ocular abnormalities. Which protein is defective?
  10. A child has blue sclerae and multiple fractures. Which protein is defective?
  11. A tall man has small testes, gynecomastia and infertility. What is his karyotype?
  12. A 12-year-old has multiple cafe-au-lait spots and axillary freckling. Which gene is mutated?
  13. A 2-month-old boy has vomiting, cataracts and jaundice after starting breast milk. Which enzyme is deficient?
  14. A woman with a family history of colon cancer has hundreds of colonic polyps at age 20. Which gene is mutated?
  15. A child has a high-pitched cry, microcephaly and intellectual disability. Which chromosome abnormality is present?
  16. An infant has urine that smells like maple syrup. Which supplement may help some patients?
  17. A child has obesity, hyperphagia and hypotonia. The deletion is on the paternal chromosome 15. What is the diagnosis?
  18. A 20-year-old man has muscle cramps and myoglobinuria after exercise, with no rise in lactate. Which enzyme is deficient?
  19. A boy has long limbs, lens dislocation upward and aortic root dilation. Which protein is defective?
  20. A 35-year-old woman has bilateral breast cancer and a family history of ovarian cancer. Which gene is most likely mutated?
  21. A man has dark urine on standing, joint pain and black ear cartilage. Which enzyme is deficient?
  22. An infant has cardiomegaly, hypotonia and a short PR interval. Which enzyme is deficient?
  23. A 60-year-old has megaloblastic anemia with hypersegmented neutrophils and neurologic findings. Which lab is elevated?
  24. A newborn has lethargy, vomiting, hyperammonemia and elevated orotic acid. Which enzyme is deficient?
  25. A woman has short stature, webbed neck, streak ovaries and primary amenorrhea. What is her karyotype?
  26. A 6-year-old has hypoglycemia on fasting, hepatomegaly and lactic acidosis. Which enzyme is deficient in this glycogen storage disease?
  27. A newborn has a cleft lip, polydactyly and holoprosencephaly. Which trisomy is most likely?
  28. A 40-year-old has chorea, dementia and caudate atrophy. Which inheritance pattern applies?
  29. A 35-year-old man has recurrent kidney stones that are hexagonal and cyanide-nitroprusside positive. What is the defect?
  30. A newborn has a musty odor, intellectual disability and fair skin. Which amino acid becomes essential?
  31. A boy has intellectual disability, large ears and macroorchidism. Which repeat expansion is present?
  32. A boy has progressive muscle weakness, calf pseudohypertrophy and a Gowers sign. Which gene is mutated?

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