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

N-acetylglutamate is made in liver mitochondria when arginine and glutamate are plentiful. It acts as an obligatory allosteric activator of which urea cycle enzyme?

AOrnithine transcarbamylaseBArginaseCArgininosuccinate 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

N-acetylglutamate is required for the first enzyme of the cycle to work at all, and rises after a protein meal when arginine is plentiful. Its deficiency therefore mimics a defect of the first enzyme and is treated with carglumic acid. The later enzymes need no such activator.

Option by option

A. Ornithine transcarbamylase
Ornithine transcarbamylase has no allosteric requirement for it.
B. Arginase
Arginase requires manganese, not N-acetylglutamate.
C. Argininosuccinate lyase
Argininosuccinate lyase cleaves argininosuccinate to arginine and fumarate without needing it.
D. Carbamoyl phosphate synthetase I (correct)
Correct. The first enzyme of the cycle is inactive without this allosteric activator.

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. N-acetylglutamate is required for the first enzyme of the cycle to work at all, and rises after a protein meal when arginine is plentiful.

Why is "Ornithine transcarbamylase" not the answer?

Ornithine transcarbamylase has no allosteric requirement for it.

Why is "Arginase" not the answer?

Arginase requires manganese, not N-acetylglutamate.

Why is "Argininosuccinate lyase" not the answer?

Argininosuccinate lyase cleaves argininosuccinate to arginine and fumarate without needing it.

What concept does this question test?

It tests Urea cycle: carbamoyl phosphate synthetase I in Biochemistry. Its deficiency therefore mimics a defect of the first enzyme and is treated with carglumic acid. The later enzymes need no such activator.

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: 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.