Which condition is associated with the inability to metabolize branched chain amino acids?

• A. Alkaptonuria
• B. Phenylketonuria
• C. Maple Syrup Disease
• D. Black Urine Disease

Answer: (C)

Metabolism of branched-chain amino acids relies on the branched-chain α-ketoacid dehydrogenase complex; a defect here blocks the breakdown of leucine, isoleucine, and valine, causing their accumulation and the distinctive signs of Maple Syrup Urine Disease. This condition is characterized by a sweet-smelling urine and neurologic symptoms if untreated, and management focuses on restricting branched-chain amino acids and sometimes using thiamine supplementation since the enzyme complex uses thiamine as a cofactor. The other listed disorders involve different steps in amino acid breakdown: alkaptonuria and its counterpart aliases arise from a defect in homogentisate oxidase in tyrosine degradation, phenylketonuria stems from deficient phenylalanine hydroxylase, and Black Urine Disease is an older name associated with alkaptonuria.