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Question Date: 27/08/2023

Question: A male newborn appears well for the first 24 hours after a term vaginal delivery but then refuses feeding and becomes irritable with rapidly progressive lethargy<br>On presentation, he is afebrile but unresponsive and experiences convulsions after which he is intubated<br>Lab studies reveal a normal WBC, a significantly elevated ammonia level, decreased plasma level of citrulline, and an elevated urine orotic acid.<br>The presentation is most consistent with a deficiency in which of the following?

Options:

• Arginase
• N-acetylglutamate synthase
• Ornithne transcarbamylase
• Arginosuccinate lyase

Correct Answer: Ornithne transcarbamylase

Explaination: Excess amino acids are metabolically consumed via the urea cycle, which serves to convert ammonia into excretable urea and synthesize arginine, which is essential for nitric oxide and creatine production. Neonates with urea cycle defects generally appear normal for their first 24 hours of life (as they benefit from the placental clearance of ammonia) and afterward become lethargic and have difficulty feeding

Reference: Kwon JM (2018). Testing for Inborn Errors of Metabolism. CONTINUUM Lifelong Learning in Neurology, 24, 3756. doi10.1212con.0000000000000563