Amino acid metabolism: maple syrup urine disease: 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: maple syrup urine disease

Sample item

A neonate has poor feeding, dystonia and urine with a sweet odour. Defective oxidative decarboxylation of which amino acids is expected?

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

The answer and the reasoning

D. Leucine, isoleucine and valine

Maple syrup urine disease is due to branched-chain alpha-ketoacid dehydrogenase deficiency. The branched-chain amino acids leucine, isoleucine and valine accumulate and are neurotoxic.

Option by option

  • A. Phenylalanine and tyrosine

    Phenylalanine hydroxylase deficiency causes phenylketonuria.

  • B. Methionine and cysteine

    Methionine metabolism defects cause homocystinuria.

  • C. Glycine and serine

    These are not substrates for the deficient branched-chain complex.

  • D. Leucine, isoleucine and valine (correct)

    Correct. These are the three branched-chain amino acids.

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 D, Leucine, isoleucine and valine. Maple syrup urine disease is due to branched-chain alpha-ketoacid dehydrogenase deficiency.

Why is "Phenylalanine and tyrosine" not the answer?

Phenylalanine hydroxylase deficiency causes phenylketonuria.

Why is "Methionine and cysteine" not the answer?

Methionine metabolism defects cause homocystinuria.

Why is "Glycine and serine" not the answer?

These are not substrates for the deficient branched-chain complex.

What concept does this question test?

It tests Amino acid metabolism: maple syrup urine disease in Biochemistry. The branched-chain amino acids leucine, isoleucine and valine accumulate and are neurotoxic.

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: maple syrup urine disease?

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.