Amino acid metabolism: hyperammonaemia treatment: 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: hyperammonaemia treatment

Sample item

A 3-day-old neonate with a suspected urea cycle disorder has a plasma ammonia of 900 micromol/L. Intravenous sodium benzoate and sodium phenylacetate are given. How do these drugs lower ammonia?

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

The answer and the reasoning

B. They conjugate with glycine and glutamine to form compounds excreted in urine, removing nitrogen

Benzoate conjugates with glycine to form hippurate, and phenylacetate conjugates with glutamine to form phenylacetylglutamine. Both are excreted in urine, so each carries away one or two nitrogen atoms by a route that bypasses the blocked urea cycle. Dialysis is added for very high levels, with protein restriction and arginine or citrulline according to the defect.

Option by option

  • A. They inhibit glutaminase in the kidney to lower ammonia production

    Inhibition of renal glutaminase is not the mechanism of these drugs.

  • B. They conjugate with glycine and glutamine to form compounds excreted in urine, removing nitrogen (correct)

    Correct. Conjugates of glycine and glutamine provide an alternative route of nitrogen excretion.

  • C. They activate carbamoyl phosphate synthetase I directly

    Direct activation of carbamoyl phosphate synthetase I is the action of carglumic acid, through N-acetylglutamate mimicry, and not of scavengers.

  • D. They bind ammonia in the gut and prevent its absorption

    Binding ammonia in the gut describes the effect of lactulose, which acidifies colonic content.

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 B, They conjugate with glycine and glutamine to form compounds excreted in urine, removing nitrogen. Benzoate conjugates with glycine to form hippurate, and phenylacetate conjugates with glutamine to form phenylacetylglutamine.

Why is "They inhibit glutaminase in the kidney to lower ammonia production" not the answer?

Inhibition of renal glutaminase is not the mechanism of these drugs.

Why is "They activate carbamoyl phosphate synthetase I directly" not the answer?

Direct activation of carbamoyl phosphate synthetase I is the action of carglumic acid, through N-acetylglutamate mimicry, and not of scavengers.

Why is "They bind ammonia in the gut and prevent its absorption" not the answer?

Binding ammonia in the gut describes the effect of lactulose, which acidifies colonic content.

What concept does this question test?

It tests Amino acid metabolism: hyperammonaemia treatment in Biochemistry. Both are excreted in urine, so each carries away one or two nitrogen atoms by a route that bypasses the blocked urea cycle. Dialysis is added for very high levels, with protein restriction and arginine or citrulline according to the defect.

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 Amino acid metabolism: hyperammonaemia treatment?

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.