A 4-year-old boy has progressive spastic diplegia with scissoring gait, developmental delay, and seizures. Plasma ammonia is mildly elevated at 120 micromol/L and plasma arginine is markedly increased. MRI shows cortical atrophy. Which urea cycle disorder fits best?

• A Citrullinemia type I
• B Carbamoyl phosphate synthetase I deficiency
• C Arginase deficiency ✓
• D Ornithine transcarbamylase deficiency

Explanation

Arginase deficiency (hyperargininemia) uniquely presents with progressive spasticity rather than neonatal hyperammonemic coma. Ammonia rises only modestly because arginine can still be degraded by other routes, and arginine itself accumulates proximal to the blocked final step. The other options present with severe neonatal or infantile hyperammonemic encephalopathy without pyramidal tract signs.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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