Urea cycle: carbamoyl phosphate synthetase 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 · Urea cycle: carbamoyl phosphate synthetase I

Sample item

A 3-day-old term infant is lethargic and feeds poorly, with a plasma ammonia several times normal. Plasma citrulline and arginine are low, urinary orotic acid is low, and a sister died in infancy. Which enzyme is most likely deficient?

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

The answer and the reasoning

C. Carbamoyl phosphate synthetase I

Low citrulline places the block at or before citrulline formation. Low orotic acid means carbamoyl phosphate is not accumulating, which separates this from ornithine transcarbamylase deficiency where orotic acid is high. An affected sister fits autosomal recessive inheritance.

Option by option

  • A. Arginase

    Arginase deficiency presents later with spastic diplegia and raised arginine, not with neonatal low citrulline.

  • B. Ornithine transcarbamylase

    Ornithine transcarbamylase deficiency is X-linked and causes high urinary orotic acid as carbamoyl phosphate spills into pyrimidine synthesis.

  • C. Carbamoyl phosphate synthetase I (correct)

    Correct. Low citrulline with low orotic acid and affected sibling fits an autosomal recessive block at the first step.

  • D. Argininosuccinate synthetase

    Argininosuccinate synthetase deficiency (citrullinaemia) raises plasma citrulline markedly.

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 C, Carbamoyl phosphate synthetase I. Low citrulline places the block at or before citrulline formation.

Why is "Arginase" not the answer?

Arginase deficiency presents later with spastic diplegia and raised arginine, not with neonatal low citrulline.

Why is "Ornithine transcarbamylase" not the answer?

Ornithine transcarbamylase deficiency is X-linked and causes high urinary orotic acid as carbamoyl phosphate spills into pyrimidine synthesis.

Why is "Argininosuccinate synthetase" not the answer?

Argininosuccinate synthetase deficiency (citrullinaemia) raises plasma citrulline markedly.

What concept does this question test?

It tests Urea cycle: carbamoyl phosphate synthetase I in Biochemistry. Low orotic acid means carbamoyl phosphate is not accumulating, which separates this from ornithine transcarbamylase deficiency where orotic acid is high. An affected sister fits autosomal recessive inheritance.

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 Urea cycle: carbamoyl phosphate synthetase 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.