A 14-year-old boy with progressive spastic diplegia and intellectual disability has plasma ammonia mildly elevated (110 micromol/L) and plasma arginine markedly elevated (600 micromol/L; normal <120). Which enzyme deficiency explains this presentation?

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

Explanation

Arginase deficiency (hyperargininemia) is unique among urea cycle defects because the block is at the final step hydrolyzing arginine to urea and ornithine. Ammonia elevation is mild or intermittent, and the dominant phenotype is progressive spasticity rather than neonatal collapse. The other options are proximal cycle defects that cause severe neonatal hyperammonemia with low arginine, not isolated hyperargininemia.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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