A 3-year-old boy from rural Maharashtra is brought to the emergency department with a 2-day history of severe vomiting, lethargy, and hypoglycemia (blood glucose 35 mg/dL). His mother reports he has not eaten for 18 hours due to gastroenteritis. On examination, he is lethargic with hepatomegaly. Serum ammonia is elevated at 180 µmol/L (normal <50). Urine organic acids show elevated dicarboxylic acids. What is the most likely enzymatic defect responsible for his clinical presentation?