A term neonate presents at 36 hours of life with poor feeding, lethargy and a seizure. Serum ammonia is markedly elevated with normal glucose and mild metabolic acidosis. Urine organic acids are unremarkable. Which enzyme deficiency is the most likely cause?

• A Carbamoyl phosphate synthetase I
• B Methylmalonyl-CoA mutase
• C Propionyl-CoA carboxylase
• D Ornithine transcarbamylase ✓

Explanation

Severe hyperammonemia with normal anion gap, respiratory alkalosis and normal urine organic acids points to a urea cycle defect, the commonest being ornithine transcarbamylase deficiency, an X-linked disorder. Organic acidemias such as propionic aciduria also cause hyperammonemia but produce marked metabolic acidosis and abnormal urine organic acids.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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