Amino acids: urea cycle: 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 acids: urea cycle

Sample item

A newborn has hyperammonemia, respiratory alkalosis, and elevated urinary orotic acid without megaloblastic anemia. Which enzyme defect is most likely?

AUMP synthase deficiencyBPhenylalanine hydroxylase deficiencyCArginase deficiency onlyDOrnithine transcarbamylase deficiency

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

The answer and the reasoning

D. Ornithine transcarbamylase deficiency

Ornithine transcarbamylase deficiency causes carbamoyl phosphate accumulation that is diverted to orotic acid, producing hyperammonemia. Unlike UMP synthase deficiency, it does not cause megaloblastic anemia.

Option by option

A. UMP synthase deficiency
UMP synthase deficiency causes orotic aciduria with megaloblastic anemia but not hyperammonemia.
B. Phenylalanine hydroxylase deficiency
Phenylalanine hydroxylase deficiency causes phenylketonuria.
C. Arginase deficiency only
Arginase deficiency usually presents later with spasticity.
D. Ornithine transcarbamylase deficiency (correct)
Correct. OTC deficiency links hyperammonemia to elevated orotic acid.

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. Ornithine transcarbamylase deficiency causes carbamoyl phosphate accumulation that is diverted to orotic acid, producing hyperammonemia.

Why is "UMP synthase deficiency" not the answer?

UMP synthase deficiency causes orotic aciduria with megaloblastic anemia but not hyperammonemia.

Why is "Phenylalanine hydroxylase deficiency" not the answer?

Phenylalanine hydroxylase deficiency causes phenylketonuria.

Why is "Arginase deficiency only" not the answer?

Arginase deficiency usually presents later with spasticity.

What concept does this question test?

It tests Amino acids: urea cycle in Biochemistry. Unlike UMP synthase deficiency, it does not cause megaloblastic anemia.

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 acids: urea cycle?

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.