Amino acid metabolism: argininosuccinic aciduria: 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: argininosuccinic aciduria

Sample item

A 6-month-old girl has episodes of vomiting and lethargy after protein-rich feeds, with ammonia 400 micromol/L. Her hair is dry and breaks easily, and microscopy shows trichorrhexis nodosa. Plasma citrulline is moderately raised and arginine is low. Which enzyme is deficient?

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

The answer and the reasoning

A. Argininosuccinate lyase

Deficiency of the lyase blocks cleavage of argininosuccinate, which accumulates and is excreted in urine, while citrulline is moderately raised and arginine falls. Brittle, tufted hair is a characteristic extra feature. Arginine becomes essential and is supplemented.

Option by option

  • A. Argininosuccinate lyase (correct)

    Correct. A block in cleaving argininosuccinate gives moderate citrulline rise, low arginine and brittle hair.

  • B. Ornithine transcarbamylase

    Deficiency of ornithine transcarbamylase gives low citrulline with raised urinary orotic acid.

  • C. Carbamoyl phosphate synthetase I

    Deficiency of carbamoyl phosphate synthetase I gives low citrulline and low urinary orotic acid.

  • D. Argininosuccinate synthetase

    The synthetase defect gives a very high citrulline level and no accumulation of the argininosuccinate intermediate.

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, Argininosuccinate lyase. Deficiency of the lyase blocks cleavage of argininosuccinate, which accumulates and is excreted in urine, while citrulline is moderately raised and arginine falls.

Why is "Ornithine transcarbamylase" not the answer?

Deficiency of ornithine transcarbamylase gives low citrulline with raised urinary orotic acid.

Why is "Carbamoyl phosphate synthetase I" not the answer?

Deficiency of carbamoyl phosphate synthetase I gives low citrulline and low urinary orotic acid.

Why is "Argininosuccinate synthetase" not the answer?

The synthetase defect gives a very high citrulline level and no accumulation of the argininosuccinate intermediate.

What concept does this question test?

It tests Amino acid metabolism: argininosuccinic aciduria in Biochemistry. Brittle, tufted hair is a characteristic extra feature. Arginine becomes essential and is supplemented.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: argininosuccinic aciduria?

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.