THEME AND FOCUS: AMINO ACIDS Case Study: A female neonate did well until approximately 24 hours of age when she became lethargic. A sepsis workup proved negative. At 54 hours, she staed showing focal seizure activity. The plasma ammonia level was found to be 1,000 mmol/L (normal 5-35 mmol/L). Quantitative plasma amino acid levels revealed a marked elevation of argininosuccinate. Lead Question: Which one of the following enzymic activities is most likely to be deficient in this patient?