Amino acid metabolism: carglumic acid: 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: carglumic acid

Sample item

A neonate has hyperammonaemia with low plasma citrulline and low urinary orotic acid. CPS I is structurally normal, but the enzyme that forms its obligatory allosteric activator from acetyl-CoA and glutamate is absent. Which drug acts as an analogue that replaces that activator?

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

The answer and the reasoning

D. Carglumic acid

Carglumic acid is a stable analogue of N-acetylglutamate and activates CPS I directly, restoring the first urea cycle step. Phenylbutyrate and benzoate give alternative nitrogen excretion but do not replace the activator.

Option by option

  • A. L-arginine

    Arginine supplies a urea cycle intermediate and stimulates formation of the activator, which cannot be made here.

  • B. Sodium phenylbutyrate

    It removes nitrogen as phenylacetylglutamine through an alternative route; it does not activate CPS I.

  • C. Lactulose

    It acidifies the colon and traps ammonia as ammonium, with no direct effect on hepatic CPS I.

  • D. Carglumic acid (correct)

    Correct. It mimics the missing activator and switches CPS I back on.

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, Carglumic acid. Carglumic acid is a stable analogue of N-acetylglutamate and activates CPS I directly, restoring the first urea cycle step.

Why is "L-arginine" not the answer?

Arginine supplies a urea cycle intermediate and stimulates formation of the activator, which cannot be made here.

Why is "Sodium phenylbutyrate" not the answer?

It removes nitrogen as phenylacetylglutamine through an alternative route; it does not activate CPS I.

Why is "Lactulose" not the answer?

It acidifies the colon and traps ammonia as ammonium, with no direct effect on hepatic CPS I.

What concept does this question test?

It tests Amino acid metabolism: carglumic acid in Biochemistry. Phenylbutyrate and benzoate give alternative nitrogen excretion but do not replace the 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 Amino acid metabolism: carglumic acid?

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.