Metabolic: urea cycle defects: practice question

A Pediatrics 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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Pediatrics · Metabolic: urea cycle defects

Sample item

A term neonate becomes lethargic with respiratory alkalosis and ammonia of 900 micromol/L after protein feeds on day 3. Orotic acid in urine is raised and citrulline is low. What is the likely diagnosis?

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

The answer and the reasoning

B. Ornithine transcarbamylase deficiency

OTC deficiency is the commonest, X-linked urea cycle disorder, with hyperammonaemia, respiratory alkalosis, high orotic acid and low citrulline. Treatment includes stopping protein, IV glucose, nitrogen scavengers such as sodium benzoate and haemodialysis. Organic acidaemias give metabolic acidosis with ketosis.

Option by option

  • A. Galactosaemia

    Galactosaemia presents with jaundice and sepsis.

  • B. Ornithine transcarbamylase deficiency (correct)

    Correct. High orotic acid with low citrulline suggests OTC deficiency.

  • C. Methylmalonic acidaemia

    Methylmalonic acidaemia causes metabolic acidosis.

  • D. Fatty acid oxidation defect

    Fatty acid oxidation defects cause hypoketotic hypoglycaemia.

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 Pediatrics question

What is the answer to this Pediatrics question?

The best answer is B, Ornithine transcarbamylase deficiency. OTC deficiency is the commonest, X-linked urea cycle disorder, with hyperammonaemia, respiratory alkalosis, high orotic acid and low citrulline.

Why is "Galactosaemia" not the answer?

Galactosaemia presents with jaundice and sepsis.

Why is "Methylmalonic acidaemia" not the answer?

Methylmalonic acidaemia causes metabolic acidosis.

Why is "Fatty acid oxidation defect" not the answer?

Fatty acid oxidation defects cause hypoketotic hypoglycaemia.

What concept does this question test?

It tests Metabolic: urea cycle defects in Pediatrics. Treatment includes stopping protein, IV glucose, nitrogen scavengers such as sodium benzoate and haemodialysis. Organic acidaemias give metabolic acidosis with ketosis.

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 Metabolic: urea cycle defects?

Memorise milestones and the vaccine schedule as tables and revise them often. Then use the flashcards and the other Pediatrics 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.