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.

Try it

Biochemistry · Urea cycle: carbamoyl phosphate synthetase I

Sample item

A child on valproate for epilepsy becomes drowsy and vomits, with high ammonia and normal liver enzymes. A valproate metabolite reduces production of N-acetylglutamate. Failure to activate which enzyme mainly explains the hyperammonaemia?

AArginaseBGlutamine synthetaseCArgininosuccinate lyaseDCarbamoyl phosphate synthetase I

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

The answer and the reasoning

D. Carbamoyl phosphate synthetase I

Valproate metabolites inhibit N-acetylglutamate synthase, so the allosteric activator of the first urea cycle enzyme falls and ammonia is not cleared. Treatment includes stopping valproate and, if needed, carglumic acid or carnitine. Later urea cycle enzymes do not depend on this activator.

Option by option

A. Arginase
Arginase is the last step and is unaffected by loss of the activator.
B. Glutamine synthetase
Glutamine synthetase detoxifies ammonia in brain and muscle and is not dependent on N-acetylglutamate.
C. Argininosuccinate lyase
Argininosuccinate lyase is a later step and does not require the activator.
D. Carbamoyl phosphate synthetase I (correct)
Correct. Without N-acetylglutamate the first urea cycle enzyme cannot incorporate ammonia.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Carbamoyl phosphate synthetase I. Valproate metabolites inhibit N-acetylglutamate synthase, so the allosteric activator of the first urea cycle enzyme falls and ammonia is not cleared.

Why is "Arginase" not the answer?

Arginase is the last step and is unaffected by loss of the activator.

Why is "Glutamine synthetase" not the answer?

Glutamine synthetase detoxifies ammonia in brain and muscle and is not dependent on N-acetylglutamate.

Why is "Argininosuccinate lyase" not the answer?

Argininosuccinate lyase is a later step and does not require the activator.

What concept does this question test?

It tests Urea cycle: carbamoyl phosphate synthetase I in Biochemistry. Treatment includes stopping valproate and, if needed, carglumic acid or carnitine. Later urea cycle enzymes do not depend on this activator.

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.