Question

Branched chain ketoacid decarboxylation is defective in:

• A. Maple syrup urine disease
• B. Hartnup disease
• C. Alkaptonuria
• D. GM1 gangliosidosis

Hint:

Defective branched chain alpha ketoacid dehydrogenase gives urine that smells of maple syrup.

Answer 1

The Correct Option is A

Step 1: Identify the metabolic step. The branched chain amino acids leucine, isoleucine and valine are first transaminated to their branched chain alpha ketoacids. The next step is oxidative decarboxylation of these ketoacids by the branched chain alpha ketoacid dehydrogenase complex.
Step 2: A deficiency of branched chain alpha ketoacid dehydrogenase causes Maple syrup urine disease (MSUD). The branched chain ketoacids and their amino acids accumulate in blood and urine, giving the urine a characteristic sweet maple syrup smell.
Step 3: Rule out the distractors. Hartnup disease (B) is a defect in the renal and intestinal transport of neutral amino acids, especially tryptophan, not branched chain ketoacid metabolism. Alkaptonuria (C) is a deficiency of homogentisate oxidase in the tyrosine pathway, causing dark urine and ochronosis. GM1 gangliosidosis (D) is a lysosomal storage disease from beta galactosidase deficiency, unrelated to amino acid catabolism.
Step 4: The only condition with a defect in branched chain ketoacid decarboxylation is Maple syrup urine disease.
Answer: Option A (Maple syrup urine disease).