A 6-month-old female infant, previously healthy, is brought to the emergency department with acute onset irritability, vomiting, and altered consciousness. The mother reports that the infant had been on a high-protein diet supplementation for 2 weeks. On examination, the child is lethargic with a characteristic 'musty' or 'mousy' odor on the breath. Laboratory findings show: blood ammonia 520 µmol/L, arterial pH 7.28 with respiratory alkalosis, plasma citrulline 1200 µmol/L (normal 20–60), and plasma arginine 80 µmol/L (normal 40–130). Urine shows elevated argininosuccinate. What is the most likely diagnosis?