A 12-year-old boy presents with progressive spastic paraplegia and mild intellectual disability. He has had only one episode of hyperammonemia in infancy. Plasma arginine is markedly elevated (650 µmol/L; normal <90). Which enzyme deficiency is responsible?

• A N-Acetylglutamate synthase
• B Argininosuccinate lyase
• C Argininosuccinate synthetase
• D Arginase ✓

Explanation

Arginase deficiency causes progressive spastic paraplegia (unique among urea cycle disorders) with relatively mild hyperammonemia and markedly elevated plasma arginine. The spasticity is attributed to arginine and its metabolites, not ammonia. Argininosuccinate lyase deficiency also elevates arginine but less dramatically and presents with hepatomegaly and trichorrhexis nodosa.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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