A homozygous infant with maple syrup urine disease (MSUD) presents at day 5 of life with poor feeding, maple syrup-smelling urine, and encephalopathy. The enzyme defective in MSUD is:
- A Branched-chain aminotransferase
- B Methylmalonyl-CoA mutase
- C Branched-chain alpha-keto acid dehydrogenase complex ✓
- D Propionyl-CoA carboxylase
Correct answer: C. Branched-chain alpha-keto acid dehydrogenase complex
Explanation
MSUD results from deficiency of the mitochondrial branched-chain alpha-keto acid dehydrogenase complex, which catalyzes the oxidative decarboxylation of alpha-keto acids derived from leucine, isoleucine, and valine. This complex requires thiamine pyrophosphate, lipoic acid, CoA, FAD, and NAD+ as cofactors. Accumulation of branched-chain amino acids and their corresponding keto acids (especially the leucine keto acid, alpha-ketoisocaproate) causes the characteristic maple syrup odor and severe neurotoxicity.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.