Amino acid metabolism: isovaleric acidaemia: 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: isovaleric acidaemia

Sample item

A 6-day-old infant develops vomiting, lethargy and metabolic acidosis after protein feeds. Nursing staff note a smell like sweaty feet. Which enzyme in the leucine pathway is deficient?

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

The answer and the reasoning

D. Isovaleryl-CoA dehydrogenase

Isovaleric acidaemia is due to deficiency of isovaleryl-CoA dehydrogenase, the third step in leucine degradation. Isovaleryl-CoA accumulates and is converted to isovaleric acid, which has the typical sweaty-feet odour, and to isovalerylglycine. Treatment uses a low-leucine diet with glycine and carnitine, which promote excretion of the conjugates.

Option by option

  • A. Homogentisate oxidase

    Homogentisate oxidase deficiency causes alkaptonuria with dark urine and arthritis.

  • B. Propionyl-CoA carboxylase

    Propionyl-CoA carboxylase deficiency leads to propionic acidaemia with different organic acids.

  • C. Medium-chain acyl-CoA dehydrogenase

    Medium-chain acyl-CoA dehydrogenase deficiency causes hypoketotic hypoglycaemia after fasting, not an odour after protein feeds.

  • D. Isovaleryl-CoA dehydrogenase (correct)

    Correct. A block in leucine catabolism raises isovaleric acid and gives the characteristic 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 D, Isovaleryl-CoA dehydrogenase. Isovaleric acidaemia is due to deficiency of isovaleryl-CoA dehydrogenase, the third step in leucine degradation.

Why is "Homogentisate oxidase" not the answer?

Homogentisate oxidase deficiency causes alkaptonuria with dark urine and arthritis.

Why is "Propionyl-CoA carboxylase" not the answer?

Propionyl-CoA carboxylase deficiency leads to propionic acidaemia with different organic acids.

Why is "Medium-chain acyl-CoA dehydrogenase" not the answer?

Medium-chain acyl-CoA dehydrogenase deficiency causes hypoketotic hypoglycaemia after fasting, not an odour after protein feeds.

What concept does this question test?

It tests Amino acid metabolism: isovaleric acidaemia in Biochemistry. Isovaleryl-CoA accumulates and is converted to isovaleric acid, which has the typical sweaty-feet odour, and to isovalerylglycine. Treatment uses a low-leucine diet with glycine and carnitine, which promote excretion of the conjugates.

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: isovaleric acidaemia?

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.