A 3-year-old boy from rural Maharashtra presents with recurrent episodes of vomiting, lethargy, and developmental delay. His mother reports that symptoms worsen after high-protein meals. On examination, he is irritable with hepatomegaly. Laboratory investigations show: ammonia 180 µmol/L (normal <50), normal liver function tests, and normal blood glucose. Plasma amino acid analysis reveals elevated glutamine and alanine. Urine orotic acid is markedly elevated. Which enzyme deficiency is most likely responsible for this clinical presentation?