A newborn with symptoms consistent with maple syrup urine disease; DNPH test shows yellow and amino acids leucine/isoleucine/valine high. What is the underlying enzyme deficiency?

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Multiple Choice

A newborn with symptoms consistent with maple syrup urine disease; DNPH test shows yellow and amino acids leucine/isoleucine/valine high. What is the underlying enzyme deficiency?

Maple syrup urine disease is caused by a deficiency of the branched-chain α-ketoacid dehydrogenase complex, the enzyme that decarboxylates the α-keto acids formed from the branched-chain amino acids leucine, isoleucine, and valine. When this complex is defective, the corresponding α-ketoacids (and the amino acids themselves) accumulate and are excreted in the urine. Those keto acids react with reagents like DNPH, producing a color change, and the build-up of leucine, isoleucine, and valine gives the urine the characteristic maple syrup odor. The pattern described—positive DNPH test with elevated branched-chain amino acids—fits MSUD perfectly because the problem is specifically within the pathway that normally decarboxylates these branched-chain α-keto acids. The other conditions involve different metabolic blocks: a glycogen storage disease from a debranching enzyme deficiency affects glycogen breakdown, phenylketonuria from phenylalanine hydroxylase deficiency disrupts phenylalanine metabolism, and alkaptonuria from homogentisic acid oxidase deficiency alters tyrosine degradation. None of these produce the selective accumulation of branched-chain amino acids and their α-keto acids with the same DNPH‑positive profile seen in maple syrup urine disease.

Maple syrup urine disease is caused by a deficiency of the branched-chain α-ketoacid dehydrogenase complex, the enzyme that decarboxylates the α-keto acids formed from the branched-chain amino acids leucine, isoleucine, and valine. When this complex is defective, the corresponding α-ketoacids (and the amino acids themselves) accumulate and are excreted in the urine. Those keto acids react with reagents like DNPH, producing a color change, and the build-up of leucine, isoleucine, and valine gives the urine the characteristic maple syrup odor. The pattern described—positive DNPH test with elevated branched-chain amino acids—fits MSUD perfectly because the problem is specifically within the pathway that normally decarboxylates these branched-chain α-keto acids.

The other conditions involve different metabolic blocks: a glycogen storage disease from a debranching enzyme deficiency affects glycogen breakdown, phenylketonuria from phenylalanine hydroxylase deficiency disrupts phenylalanine metabolism, and alkaptonuria from homogentisic acid oxidase deficiency alters tyrosine degradation. None of these produce the selective accumulation of branched-chain amino acids and their α-keto acids with the same DNPH‑positive profile seen in maple syrup urine disease.

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