Urea cycle: ornithine transcarbamylase: 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: ornithine transcarbamylase

Sample item

A newborn boy has hyperammonaemia, low blood urea nitrogen and elevated urinary orotic acid without megaloblastic anaemia. Which defect is most likely?

ACarbamoyl phosphate synthetase II deficiencyBArginase deficiencyCUMP synthase deficiencyDOrnithine transcarbamylase deficiency

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

The answer and the reasoning

D. Ornithine transcarbamylase deficiency

OTC deficiency is X-linked and causes excess mitochondrial carbamoyl phosphate to enter pyrimidine synthesis, raising orotic acid. UMP synthase deficiency instead causes megaloblastic anaemia without hyperammonaemia.

Option by option

A. Carbamoyl phosphate synthetase II deficiency
CPS II is cytosolic and part of de novo pyrimidine synthesis.
B. Arginase deficiency
Arginase deficiency is a later urea-cycle defect with hyperargininaemia.
C. UMP synthase deficiency
UMP synthase deficiency causes isolated hereditary orotic aciduria with megaloblastic anaemia.
D. Ornithine transcarbamylase deficiency (correct)
Correct. OTC deficiency gives hyperammonaemia plus orotic aciduria.

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, Ornithine transcarbamylase deficiency. OTC deficiency is X-linked and causes excess mitochondrial carbamoyl phosphate to enter pyrimidine synthesis, raising orotic acid.

Why is "Carbamoyl phosphate synthetase II deficiency" not the answer?

CPS II is cytosolic and part of de novo pyrimidine synthesis.

Why is "Arginase deficiency" not the answer?

Arginase deficiency is a later urea-cycle defect with hyperargininaemia.

Why is "UMP synthase deficiency" not the answer?

UMP synthase deficiency causes isolated hereditary orotic aciduria with megaloblastic anaemia.

What concept does this question test?

It tests Urea cycle: ornithine transcarbamylase in Biochemistry. UMP synthase deficiency instead causes megaloblastic anaemia without hyperammonaemia.

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: ornithine transcarbamylase?

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.