Urea cycle disorders: arginase deficiency: 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 disorders: arginase deficiency

Sample item

A 3-year-old child has progressive spastic diplegia, poor growth and occasional mild hyperammonaemic episodes. Plasma arginine is markedly raised. Which enzyme is deficient?

ACarbamoyl phosphate synthetase IBArgininosuccinate lyaseCArgininosuccinate synthetaseDArginase 1

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

The answer and the reasoning

D. Arginase 1

Arginase 1 catalyses the final cycle step, splitting arginine into urea and ornithine. Its deficiency causes hyperargininaemia, in which ammonia is only mildly raised because much waste nitrogen is already incorporated into arginine, which can be excreted, and the dominant feature is progressive spastic paraplegia from neurotoxic guanidino compounds. Argininosuccinic aciduria, citrullinaemia and proximal defects present early with severe hyperammonaemia.

Option by option

A. Carbamoyl phosphate synthetase I
Carbamoyl phosphate synthetase I deficiency gives severe neonatal hyperammonaemia with low citrulline and arginine.
B. Argininosuccinate lyase
Argininosuccinate lyase deficiency raises argininosuccinate and causes brittle hair with early hyperammonaemia.
C. Argininosuccinate synthetase
Argininosuccinate synthetase deficiency causes citrullinaemia with a high plasma citrulline.
D. Arginase 1 (correct)
Correct. A block at the last step raises arginine and gives spastic diplegia with mild ammonia elevation.

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, Arginase 1. Arginase 1 catalyses the final cycle step, splitting arginine into urea and ornithine.

Why is "Carbamoyl phosphate synthetase I" not the answer?

Carbamoyl phosphate synthetase I deficiency gives severe neonatal hyperammonaemia with low citrulline and arginine.

Why is "Argininosuccinate lyase" not the answer?

Argininosuccinate lyase deficiency raises argininosuccinate and causes brittle hair with early hyperammonaemia.

Why is "Argininosuccinate synthetase" not the answer?

Argininosuccinate synthetase deficiency causes citrullinaemia with a high plasma citrulline.

What concept does this question test?

It tests Urea cycle disorders: arginase deficiency in Biochemistry. Its deficiency causes hyperargininaemia, in which ammonia is only mildly raised because much waste nitrogen is already incorporated into arginine, which can be excreted, and the dominant feature is progressive spastic paraplegia from neurotoxic guanidino compounds. Argininosuccinic aciduria, citrullinaemia and proximal defects present early with severe hyperammonaemia.

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 disorders: arginase deficiency?

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.