Practice Test GeeksNBME Practice Test
← All NBME Flashcard Decks

Biochemistry Flashcards

7 cards from real NBME practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Biochemistry flashcards as text

  1. Beta-oxidation of an 18-carbon fully saturated fatty acid yields how many acetyl-CoA molecules?

    Answer: 9

    An 18-carbon fatty acid undergoes 8 rounds of beta-oxidation, producing 9 acetyl-CoA molecules (each 2-carbon unit released as acetyl-CoA).

  2. A 3-month-old presents with maple syrup-smelling urine, lethargy, and elevated branched-chain amino acids (leucine, isoleucine, valine). The deficient enzyme complex is:

    Answer: Branched-chain alpha-ketoacid dehydrogenase (BCKAD)

    Maple syrup urine disease is caused by BCKAD deficiency, leading to accumulation of branched-chain amino acids and their toxic keto-acids.

  3. A patient with homocystinuria due to cystathionine beta-synthase deficiency presents with lens dislocation and thromboembolism. High-dose supplementation with which vitamin may reduce homocysteine levels in responsive patients?

    Answer: Pyridoxine (B6)

    Cystathionine beta-synthase requires PLP (vitamin B6) as a cofactor; high-dose B6 increases residual enzyme activity in responsive patients.

  4. Orotic aciduria and hyperammonemia occur in a child with an X-linked urea cycle defect. Which enzyme is deficient?

    Answer: Ornithine transcarbamylase (OTC)

    OTC deficiency causes accumulation of carbamoyl phosphate, which enters the pyrimidine synthesis pathway and is converted to orotic acid.

  5. The committed and rate-limiting step of de novo fatty acid synthesis is catalyzed by which enzyme?

    Answer: Acetyl-CoA carboxylase

    Acetyl-CoA carboxylase (requiring biotin) converts acetyl-CoA to malonyl-CoA, the committed, rate-limiting step of fatty acid synthesis.

  6. Carnitine is required for transport of long-chain fatty acids across which membrane?

    Answer: Inner mitochondrial membrane

    Long-chain fatty acids are converted to acylcarnitine to cross the inner mitochondrial membrane via the carnitine shuttle for beta-oxidation.

  7. De novo purine synthesis builds the purine ring on which activated sugar scaffold?

    Answer: PRPP (5-phosphoribosyl-1-pyrophosphate)

    PRPP (5-phosphoribosyl-1-pyrophosphate) is the activated ribose-5-phosphate substrate on which the purine ring is assembled step-by-step.