Urea cycle disorders and hyperammonaemia — formative SAQs

SAQ 1 (10 marks)

A term male infant is well at birth but at 48 hours of age becomes lethargic, feeds poorly, vomits, and is found hyperventilating with a respiratory rate of 80. Blood gas shows a respiratory alkalosis. Sepsis cultures are pending. A bedside clinician sends an ammonia, which returns at 620 micromoles per litre. ^{[1] [4]}

a) Explain why this picture of a well neonate deteriorating 24 to 72 hours into feeds with a respiratory alkalosis is a classic urea cycle disorder presentation, and list the four immediate resuscitation moves you make. (3 marks) ^{[1] [4]}

b) Describe the first-tier metabolic panel you order to localise the defect, and explain how plasma amino acids and urinary orotic acid distinguish ornithine transcarbamylase deficiency from carbamoyl phosphate synthetase 1 and NAGS deficiency. (3 marks) ^[1]

c) Outline the role of nitrogen scavengers, arginine, and extracorporeal removal in this child, including the ammonia thresholds that would prompt dialysis. (2 marks) ^[5]

d) Discuss the prognostic significance of the peak ammonia and time to treatment, and the implications for counselling this family. (2 marks) ^[6]