Urea cycle: 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

Sample item

Which enzyme catalyses the rate-limiting step of the urea cycle?

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

The answer and the reasoning

D. Carbamoyl phosphate synthetase I

Carbamoyl phosphate synthetase I, in liver mitochondria, forms carbamoyl phosphate from ammonia and bicarbonate. It is activated by N-acetylglutamate. Ornithine transcarbamylase deficiency is the commonest inherited urea cycle disorder and, unlike CPS I deficiency, gives raised orotic acid.

Option by option

  • A. Ornithine transcarbamylase

    Ornithine transcarbamylase is the next enzyme and is the most commonly deficient one, but it is not rate-limiting.

  • B. Argininosuccinate lyase

    Argininosuccinate lyase acts later in the cycle.

  • C. Arginase

    Arginase makes urea from arginine at the final step.

  • D. Carbamoyl phosphate synthetase I (correct)

    Correct. It commits ammonia to the cycle and is activated by N-acetylglutamate.

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, Carbamoyl phosphate synthetase I. Carbamoyl phosphate synthetase I, in liver mitochondria, forms carbamoyl phosphate from ammonia and bicarbonate.

Why is "Ornithine transcarbamylase" not the answer?

Ornithine transcarbamylase is the next enzyme and is the most commonly deficient one, but it is not rate-limiting.

Why is "Argininosuccinate lyase" not the answer?

Argininosuccinate lyase acts later in the cycle.

Why is "Arginase" not the answer?

Arginase makes urea from arginine at the final step.

What concept does this question test?

It tests Urea cycle in Biochemistry. It is activated by N-acetylglutamate. Ornithine transcarbamylase deficiency is the commonest inherited urea cycle disorder and, unlike CPS I deficiency, gives raised orotic acid.

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 Urea cycle?

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.