Most common genentic enzyme defect in urea cycle is: (A) Arginase, (B) Arginosuccunate lyase, (C) Ornithine transcarbomylase, (D) Carbomoyl phosphate synthase I
Answer is A. OTC combines carbamyl phosphate with ornithine to make citrulline which is subsequently processed tourea (see: cycle diagram). Along with CPSI and NAGS deficiency, OTCdeficiency is the most severe of theurea cycle disorders. Patients with complete OTC deficiency rapidly develop hyperammonemia in the newborn period Ref-Harpers illustrated biochemistry 30/e p583