Adult Urea Cycle Disorder — RACP Adult Medicine MCQ

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HardGeneral Internal MedicineAdult Urea Cycle DisorderRACP Adult Medicine

A 55-year-old man with chronic alcohol use presents with confusion, asterixis, and a blood ammonia level of 130 µmol/L. He has no known liver disease. CT brain is normal. His liver function tests are surprisingly normal. What alternative cause of hyperammonaemia should be considered?

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Correct answer: A — Urea cycle disorder

While chronic liver disease is the most common cause of hyperammonaemia in adults, late-onset (adult) urea cycle disorders (most commonly ornithine transcarbamylase deficiency – OTC, X-linked) can present with episodic hyperammonaemic encephalopathy triggered by physiological stress (illness, high protein intake, medications like valproate). Normal liver function tests distinguish this from hepatic encephalopathy. Plasma amino acids (elevated glutamine, low citrulline in OTC deficiency) and urine orotic acid are diagnostic. Treatment includes nitrogen scavengers (sodium benzoate, phenylbutyrate) and dietary protein restriction.

Reference: eTG – 2025 – Gastroenterology; AMH – 2025 – Urea Cycle Disorders