A neonate has coma with ammonia of 1400 micromol/L on day 2. Plasma citrulline is undetectable, plasma arginine is low, and urine organic acid screen shows NO increase in orotic acid. Which single finding most reliably separates this condition from ornithine transcarbamylase deficiency?
- A Detectable urinary citrate
- B Elevated urinary orotic acid
- C Normal urinary orotic acid ✓
- D Presence of urinary uracil
Correct answer: C. Normal urinary orotic acid
Explanation
Both CPS-I deficiency and OTC deficiency give profound neonatal hyperammonemia with undetectable citrulline. The discriminator is orotic acid: in OTC deficiency, mitochondrial carbamoyl phosphate accumulates and spills into the cytosolic pyrimidine pathway, raising urinary orotic acid. With a CPS-I defect no carbamoyl phosphate is made at all, so urinary orotic acid remains normal. This single test point guides enzyme confirmation before molecular studies.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.