NBME Biochemistry 3

Question 1

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

Accepted Answer

9

Answer

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:

Accepted Answer

Branched-chain alpha-ketoacid dehydrogenase (BCKAD)

Answer

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?

Accepted Answer

Pyridoxine (B6)

Answer

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?

Accepted Answer

Ornithine transcarbamylase (OTC)

Answer

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?

Accepted Answer

Acetyl-CoA carboxylase

Answer

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

NBME Practice Test

NBME Practice Test

NBME Biochemistry 3