A 6-month-old female infant, born to consanguineous parents, is brought to the pediatric ward with a 2-week history of developmental regression, poor feeding, and recurrent vomiting. On examination, she is irritable with mild hepatomegaly. Investigations show: blood ammonia 420 µmol/L, plasma citrulline 8 µmol/L (normal 20–40), plasma argininosuccinate 2 µmol/L (normal <2), plasma arginine 25 µmol/L (normal 40–130), and normal urinary orotic acid. What enzyme deficiency is most likely responsible for this clinical presentation?