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Urea cycle disorder — RACP Paediatrics MCQ

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HardGenetics and metabolicUrea cycle disorderRACP Paediatrics

A 5-day-old term infant has poor feeding, vomiting and progressive lethargy. Blood gas shows respiratory alkalosis, glucose is normal and ammonia is markedly elevated. There is no evidence of sepsis. What is the most likely cause?

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Correct answer: D — Impaired ammonia detoxification from a urea cycle defect

Marked hyperammonaemia with respiratory alkalosis in an otherwise term neonate suggests a urea cycle disorder. Diabetic ketoacidosis would cause hyperglycaemia and metabolic acidosis, not isolated ammonia toxicity. Urgent metabolic treatment and diagnostic plasma amino acids and urine orotic acid are needed.

Reference: RACP Knowledge Guides; Australian metabolic emergency guidance