An 18-month-old boy evaluated for progressive gait abnormality is found to have bilateral spastic diplegia with preserved intellect. Ammonia is intermittently mildly elevated (90 micromol/L). Plasma shows high arginine and low-normal citrulline. Urinary orotic acid is normal. Which enzyme deficiency explains this presentation?
- A Ornithine transcarbamylase
- B Argininosuccinate synthetase
- C Arginase ✓
- D N-acetylglutamate synthase
Correct answer: C. Arginase
Explanation
Arginase deficiency (hyperargininemia) is the atypical urea cycle defect: it presents after infancy with progressive spastic diplegia rather than neonatal hyperammonemic crisis, because residual hepatic arginase activity limits ammonia buildup. Elevated plasma arginine with normal urinary orotic acid distinguishes it from OTC deficiency. Citrulline rises in argininosuccinate synthetase deficiency, and NAGS deficiency produces severe neonatal hyperammonemia with low all cycle intermediates.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.