Amino acid metabolism: citrullinaemia type I: 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: citrullinaemia type I

Sample item

A 4-day-old neonate becomes lethargic and starts to hyperventilate with plasma ammonia of 1100 micromol/L. Amino acid analysis shows plasma citrulline many times above normal with low arginine. The father and mother are healthy and first cousins. Which enzyme is deficient?

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

The answer and the reasoning

A. Argininosuccinate synthetase

Argininosuccinate synthetase condenses citrulline with aspartate. When it is deficient, citrulline accumulates to very high levels in plasma and ammonia rises. Inheritance is autosomal recessive, unlike X-linked ornithine transcarbamylase deficiency.

Option by option

  • A. Argininosuccinate synthetase (correct)

    Correct. Failure to combine citrulline with aspartate makes citrulline accumulate massively.

  • B. Ornithine transcarbamylase

    Deficiency of this enzyme causes low citrulline because its substrate cannot be converted.

  • C. Argininosuccinate lyase

    A lyase defect causes a moderate rise of citrulline with a characteristic urinary intermediate and brittle hair.

  • D. Arginase

    Arginase deficiency raises arginine and presents later with spastic diplegia rather than neonatal coma.

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 synthetase. Argininosuccinate synthetase condenses citrulline with aspartate.

Why is "Ornithine transcarbamylase" not the answer?

Deficiency of this enzyme causes low citrulline because its substrate cannot be converted.

Why is "Argininosuccinate lyase" not the answer?

A lyase defect causes a moderate rise of citrulline with a characteristic urinary intermediate and brittle hair.

Why is "Arginase" not the answer?

Arginase deficiency raises arginine and presents later with spastic diplegia rather than neonatal coma.

What concept does this question test?

It tests Amino acid metabolism: citrullinaemia type I in Biochemistry. When it is deficient, citrulline accumulates to very high levels in plasma and ammonia rises. Inheritance is autosomal recessive, unlike X-linked ornithine transcarbamylase deficiency.

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: citrullinaemia type I?

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.