A 7-year-old boy has progressive spastic diplegia with preserved intellect, short stature, and recurrent episodes of lethargy after high-protein meals. Plasma ammonia is mildly elevated at 120 micromol/L, plasma arginine is markedly raised, and urine orotic acid is normal. The most likely enzyme defect is:

• A Arginase I ✓
• B Argininosuccinate synthetase
• C Ornithine transcarbamoylase
• D Carbamoyl phosphate synthetase-I

Explanation

Arginase deficiency (hyperargininemia) is the most indolent urea cycle disorder: the block is distal, so ammonia accumulates slowly and the dominant picture is progressive spasticity rather than neonatal crisis. Marked hyperargininemia with normal orotic acid distinguishes it from proximal blocks. In CPS-I or OTC deficiency, orotic acid rises (OTC) or citrulline falls (CPS-I), and both present with severe neonatal hyperammonemic encephalopathy.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.