NBME Biochemistry 3 — Questions and Answers
Question 1: Beta-oxidation of an 18-carbon fully saturated fatty acid yields how many acetyl-CoA molecules?
- 7
- 8
- 9 (Correct answer)
- 10
Correct 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).
Question 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:
- Phenylalanine hydroxylase
- Branched-chain alpha-ketoacid dehydrogenase (BCKAD) (Correct answer)
- Propionyl-CoA carboxylase
- Cystathionine beta-synthase
Correct 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.
Question 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?
- Thiamine (B1)
- Riboflavin (B2)
- Pyridoxine (B6) (Correct answer)
- Cobalamin (B12)
Correct answer: Pyridoxine (B6)
Cystathionine beta-synthase requires PLP (vitamin B6) as a cofactor; high-dose B6 increases residual enzyme activity in responsive patients.
Question 4: Orotic aciduria and hyperammonemia occur in a child with an X-linked urea cycle defect. Which enzyme is deficient?
- Carbamoyl phosphate synthetase I
- Argininosuccinate synthetase
- Ornithine transcarbamylase (OTC) (Correct answer)
- Arginase
Correct answer: Ornithine transcarbamylase (OTC)
OTC deficiency causes accumulation of carbamoyl phosphate, which enters the pyrimidine synthesis pathway and is converted to orotic acid.
Question 5: The committed and rate-limiting step of de novo fatty acid synthesis is catalyzed by which enzyme?
- ATP-citrate lyase
- Acetyl-CoA carboxylase (Correct answer)
- Fatty acid synthase
- Malonyl-CoA decarboxylase
Correct answer: Acetyl-CoA carboxylase
Acetyl-CoA carboxylase (requiring biotin) converts acetyl-CoA to malonyl-CoA, the committed, rate-limiting step of fatty acid synthesis.
Question 6: Carnitine is required for transport of long-chain fatty acids across which membrane?
- Outer mitochondrial membrane
- Inner mitochondrial membrane (Correct answer)
- Endoplasmic reticulum membrane
- Peroxisomal membrane
Correct 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.
Question 7: De novo purine synthesis builds the purine ring on which activated sugar scaffold?
- UDP-glucose
- CDP-ribose
- PRPP (5-phosphoribosyl-1-pyrophosphate) (Correct answer)
- dAMP
Correct 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.
NBME Biochemistry 3
Beta-oxidation of an 18-carbon fully saturated fatty acid yields how many acetyl-CoA molecules?