18 一位3個月大的嬰兒因抽搐、嘔吐、意識不清送來急診處，檢查血液發現高血氨（hyperammonemia）、血中瓜胺酸（citrulline）低下，尿中orotic acid亦降低。該病患體內最可能缺乏何者酵素？

Carbamylphosphate synthetase（CPS） Ornithine transcarbamylase（OTC）

Argininosuccinate synthetase Argininosuccinate lyase

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CPS: DR CHIONG MAKES ME VOMIT

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Carbamoyl phosphate synthetase (CPS) deficiency is a urea cycle defect that results from a deficiency in an enzyme that mediates the normal path for incorporation of ammonia

Two hepatocellular enzymes exist: CPS I and CPS II. CPS I is exclusively intramitochondrial, and its deficiency is responsible for the disease. CPS I is the most plentiful single protein in hepatic mitochondria, accounting for about 20% of the matrix protein. CPS II is exclusively cytosolic and is an important enzyme in de novo synthesis of pyrimidine nucleotides. The regulation of CPS I activity depends on the levels of N -acetylglutamate (see N-Acetylglutamate Synthetase (NAGS) Deficiency).