Amino acid metabolism: branched-chain: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Amino acid metabolism: branched-chain

Sample item

A 7-day-old breastfed neonate becomes lethargic and feeds poorly, and has a sweet, maple syrup odour in the urine and ketonuria. Which enzyme complex is defective?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Branched-chain alpha-ketoacid dehydrogenase

In maple syrup urine disease, branched-chain alpha-ketoacid dehydrogenase is deficient, so leucine, isoleucine and valine and their ketoacids accumulate. Leucine is neurotoxic. Treatment is a diet restricted in branched-chain amino acids, and some patients respond to thiamine.

Option by option

  • A. Branched-chain alpha-ketoacid dehydrogenase (correct)

    Correct. The block affects leucine, isoleucine and valine metabolism and causes the odour and encephalopathy.

  • B. Homogentisate oxidase

    Homogentisate oxidase deficiency causes alkaptonuria with dark urine on standing.

  • C. Cystathionine beta-synthase

    Cystathionine beta-synthase deficiency causes homocystinuria with lens dislocation and thrombosis.

  • D. Phenylalanine hydroxylase

    Phenylalanine hydroxylase deficiency causes phenylketonuria with a mousy odour.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Branched-chain alpha-ketoacid dehydrogenase. In maple syrup urine disease, branched-chain alpha-ketoacid dehydrogenase is deficient, so leucine, isoleucine and valine and their ketoacids accumulate.

Why is "Homogentisate oxidase" not the answer?

Homogentisate oxidase deficiency causes alkaptonuria with dark urine on standing.

Why is "Cystathionine beta-synthase" not the answer?

Cystathionine beta-synthase deficiency causes homocystinuria with lens dislocation and thrombosis.

Why is "Phenylalanine hydroxylase" not the answer?

Phenylalanine hydroxylase deficiency causes phenylketonuria with a mousy odour.

What concept does this question test?

It tests Amino acid metabolism: branched-chain in Biochemistry. Leucine is neurotoxic. Treatment is a diet restricted in branched-chain amino acids, and some patients respond to thiamine.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Amino acid metabolism: branched-chain?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.