A 5-year-old boy presents with progressive spastic diplegia, intellectual disability, and seizures. Ammonia is mildly elevated at 120 micromol/L. Plasma amino acid analysis shows arginine at 450 micromol/L (normal <120). The most likely diagnosis is:

• A Arginase deficiency ✓
• B Citrullinemia type I
• C OTC deficiency
• D Argininosuccinate lyase deficiency

Explanation

Arginase, the final urea cycle enzyme, hydrolyzes arginine to urea and ornithine. Deficiency produces a distinctive phenotype: hyperargininemia with pyramidal tract signs (spasticity) rather than neonatal hyperammonemic crisis, because residual cycling capacity limits ammonia buildup. The other listed defects present earlier with severe hyperammonemia and encephalopathy but without isolated arginine elevation or prominent spasticity as the leading sign.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.