Urea Cycle Disorders

Prepared and reviewed by the PrepElephant Academic Review Team

First published 14 Oct 2026 · Spot an error?

Reference basis: Nelson Textbook of Pediatrics; Ghai Essential Pediatrics

On this page

Direct answer
What you must remember
A metabolic emergency pathway for the neonate
Where the exam tests urea cycle disorders
Frequently asked questions
Related topics

Direct answer

Urea cycle disorders are inherited deficiencies of the hepatic enzymes or transporters that convert toxic ammonia into urea. The commonest, ornithine transcarbamylase deficiency, is X-linked. Affected neonates usually present in the first days to weeks of life, often after protein feeds begin, with vomiting, lethargy and cerebral oedema. Hyperammonaemia with primary respiratory alkalosis is the classic combination, and emergency treatment stops protein, gives calories and removes ammonia.

What you must remember

Six defects: carbamoyl phosphate synthetase 1 (CPS1), ornithine transcarbamylase (OTC), argininosuccinate synthetase (ASS1, citrullinaemia type I), argininosuccinate lyase (ASL), arginase 1 (ARG1) and N-acetylglutamate synthase (NAGS) deficiency.
Acid-base clue: primary respiratory alkalosis from hyperventilation, driven by raised ammonia stimulating the respiratory centre.
Orotic aciduria: raised urinary orotic acid is characteristic of OTC deficiency and is low in CPS1 and NAGS deficiency.
Amino acid clues: raised plasma citrulline in ASS1 deficiency, and raised argininosuccinic acid in ASL deficiency.
Sampling rule: ammonia must be collected on ice and analysed quickly, since delay falsely raises the result.
Valproate warning: valproate can precipitate hyperammonaemia in carriers of these defects, so it is avoided when a urea cycle defect is suspected.
Long-term care: lifelong low-protein diet with arginine or citrulline as the defect requires, and liver transplantation in selected cases.

A metabolic emergency pathway for the neonate

A neonate with vomiting and lethargy after feeds is treated as a metabolic emergency until proven otherwise. The catabolic state keeps ammonia production high, so calories and ammonia removal must run together.

Stop protein briefly: withhold protein intake only for as long as needed, because continuing it feeds ammonia production.
Give calories: IV glucose and lipids reverse catabolism and reduce the breakdown of body protein that releases more nitrogen.
Lower ammonia with scavengers: sodium benzoate and sodium phenylacetate, or oral sodium phenylbutyrate, bind nitrogen and excrete it through alternative pathways.
Remove ammonia with dialysis: haemodialysis is used for severe or refractory hyperammonaemia, since it clears ammonia faster than drugs alone.
Replace the missing product: arginine or citrulline is given as the defect requires, and carglumic acid is used in NAGS deficiency, where it replaces the missing cofactor.
Plan the long term: protein restriction, an emergency plan for intercurrent illness and monitoring of amino acids continue for life.

Where the exam tests urea cycle disorders

Questions focus on the acid-base pattern, the enzyme-specific markers and the drug hazards.

Trap: trusting a high ammonia from a delayed or warm sample. Correct: samples must go on ice and be analysed quickly, so a properly collected repeat sample is needed before diagnosis.
Trap: using ammonia-lowering drugs alone without stopping protein or giving calories. Correct: the catabolic state keeps ammonia high, so calories and protein restriction are needed alongside scavengers.
Trap: choosing valproate for seizures in a child with a suspected urea cycle defect. Correct: valproate precipitates hyperammonaemia in these children, so a safer antiepileptic is chosen.

Frequently asked questions

Which urea cycle disorder is the most common?

Ornithine transcarbamylase deficiency is the commonest urea cycle disorder and is X-linked. Heterozygous females can also develop symptoms, so the diagnosis should be considered in women with unexplained hyperammonaemia.

Why does a urea cycle disorder cause respiratory alkalosis?

Hyperammonaemia stimulates the respiratory centre, causing hyperventilation and primary respiratory alkalosis. This acid-base pattern is a classic clue in neonatal hyperammonaemia and points towards a urea cycle defect.

Which drugs lower ammonia in an acute urea cycle disorder?

Sodium benzoate and sodium phenylacetate, given intravenously, are the ammonia scavengers used in acute urea cycle disorders. Oral sodium phenylbutyrate is used for long-term maintenance in suitable patients.

What is the role of carglumic acid in urea cycle disorders?

Carglumic acid is an analogue of N-acetylglutamate that activates carbamoyl phosphate synthetase 1. It is used in N-acetylglutamate synthase deficiency, where it replaces the missing cofactor, and in some other forms.