Practice quiz · by a Quizcard user

USMLE Step 1 Biochemistry: Vitamins, Enzyme Deficiencies and Genetic Disorders — quiz

15 multiple-choice questions covering Fat-soluble vitamins, Vitamin K, Thiamine (B1), Niacin (B3), Folate vs B12. One answer each; you get a score at the end and the explanation for every question. No account needed.

Updated 2026-11-04. Keys 1–4 pick an answer, Enter moves on. Redo it any time, in order or shuffled.

The questions

Answers and explanations are shown after you finish.

1. Fat-soluble vitamins

   A pregnant woman is prescribed isotretinoin, a vitamin A derivative, for severe acne. Which adverse effect is the primary concern with this excess vitamin A exposure?

   • Teratogenicity
   • Hypercalcemia and kidney stones
   • Hemorrhage due to decreased clotting factor activation
   • Enhanced anticoagulant effect of warfarin
2. Vitamin K

   Vitamin K deficiency causes a bleeding diathesis primarily by impairing which biochemical process?

   • Synthesis of coenzyme A for fatty acid metabolism
   • Reduction of Fe3+ to Fe2+ for absorption
   • Gamma-carboxylation of clotting factors II, VII, IX, and X
   • Hydroxylation of proline and lysine residues
3. Thiamine (B1)

   In a chronic alcoholic with suspected thiamine deficiency, why is thiamine given before administering glucose?

   • Giving glucose first can rapidly deplete remaining thiamine stores and precipitate Wernicke encephalopathy
   • Glucose increases tryptophan diversion, worsening pellagra
   • Glucose inhibits pyridoxine kinase, worsening peripheral neuropathy
   • Glucose displaces vitamin B12 from intrinsic factor, causing anemia
4. Niacin (B3)

   Pellagra can develop in Hartnup disease because of decreased intestinal and renal absorption of which amino acid needed for niacin synthesis?

   • Methionine
   • Tyrosine
   • Phenylalanine
   • Tryptophan
5. Folate vs B12

   A patient has macrocytic anemia. Which additional clinical finding points to vitamin B12 deficiency rather than folate deficiency?

   • Subacute combined degeneration with dorsal column and corticospinal tract involvement
   • Absence of hypersegmented neutrophils
   • Normal mean corpuscular volume
   • Elevated risk of neural tube defects in offspring
6. Lysosomal storage diseases

   Which finding helps distinguish Niemann-Pick disease from Tay-Sachs disease on clinical presentation?

   • Presence of a cherry-red macular spot
   • Hepatosplenomegaly
   • Progressive neurodegeneration
   • Autosomal recessive inheritance pattern
7. Fabry disease

   Which lysosomal storage disease is notable for being X-linked recessive rather than autosomal recessive like most others in its category?

   • Krabbe disease
   • Niemann-Pick disease
   • Fabry disease
   • Gaucher disease
8. Glycogen storage diseases

   An infant presents with severe fasting hypoglycemia, lactic acidosis, hepatomegaly, and hyperuricemia. Deficiency of which enzyme is most consistent with this presentation?

   • Debranching enzyme (alpha-1,6-glucosidase)
   • Lysosomal alpha-1,4-glucosidase
   • Skeletal muscle glycogen phosphorylase
   • Glucose-6-phosphatase
9. McArdle disease

   A young adult develops painful muscle cramps and myoglobinuria after strenuous exercise but has normal fasting blood glucose. Which enzyme deficiency best explains this?

   • Skeletal muscle glycogen phosphorylase
   • Glucose-6-phosphatase
   • Lysosomal acid maltase
   • Debranching enzyme
10. Phenylketonuria

    Dietary management of phenylketonuria requires restricting phenylalanine while supplementing which amino acid that can no longer be produced endogenously?

    • Tyrosine
    • Tryptophan
    • Methionine
    • Cysteine
11. Urea cycle disorders

    A male infant has hyperammonemia, elevated orotic acid, and decreased BUN with no megaloblastic anemia. Which enzyme deficiency is most likely?

    • UMP synthase
    • Ornithine transcarbamylase
    • Adenosine deaminase
    • HGPRT
12. Purine/pyrimidine disorders

    A young boy presents with gout, intellectual disability, dystonia, and compulsive self-mutilation. Which enzyme deficiency is responsible?

    • Adenosine deaminase
    • Ornithine transcarbamylase
    • UMP synthase
    • HGPRT
13. Marfan syndrome

    In Marfan syndrome, lens subluxation typically occurs in which direction, helping distinguish it from homocystinuria?

    • Directly posterior
    • No lens involvement occurs
    • Downward and nasally
    • Upward and temporally
14. DNA repair defects

    Xeroderma pigmentosum results from a defect in which DNA repair pathway, leading to failure to correct UV-induced thymidine dimers?

    • Double-strand break repair
    • Mismatch repair
    • Nucleotide excision repair
    • Base excision repair
15. Trinucleotide repeat disorders

    Which trinucleotide repeat disorder is notable for genetic anticipation that is especially pronounced when inherited from the father?

    • Myotonic dystrophy
    • Friedreich ataxia
    • Huntington disease
    • Fragile X syndrome