USMLE Step 1 · USMLE Step 1: Medical Sciences

Biochemistry, Molecular Genetics and Cellular Biology

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

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All 6 questions

  1. A 3-year-old boy presents with developmental delay, coarse facial features, corneal clouding, and hepatosplenomegaly. Laboratory evaluation reveals deficient phosphorylation of mannose residues on gly…
  2. Which of the following enzymes in DNA replication possesses exonuclease activity in prokaryotes (E. coli), essential for removing RNA primers during Okazaki fragment maturation?
  3. A 24-year-old medical student runs a marathon. At hour 3 of the race, which rate-limiting enzyme of glycogenolysis is allosterically activated by AMP in skeletal muscle?
  4. A 4-month-old infant is brought to the clinic with severe hypoglycemia, lactic acidosis, hyperuricemia, and hepatomegaly with doll-like facies. Fasting blood glucose fails to rise after administration…
  5. A 5-year-old child presents with extreme photosensitivity, severe sunburns with minimal sun exposure, freckling, and early development of basal cell carcinoma. This autosomal recessive condition is ca…
  6. A 35-year-old woman presents with chorea, dementia, and behavioral changes. Her father died of a similar neurological disorder at age 48. Genetic testing confirms Huntington disease. The molecular bas…

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