A 7-year-old boy from rural Maharashtra presents to the pediatric clinic with a 6-month history of progressive developmental delay, learning difficulties, and episodic vomiting after high-protein meals. His mother reports that he becomes irritable and confused after eating chicken or eggs. Physical examination reveals mild hepatomegaly and no splenomegaly. Serum ammonia is 120 µmol/L (normal <50), and plasma amino acid analysis shows elevated glutamine and alanine with low citrulline. Urine chromatography shows elevated orotic acid. Which of the following is the most appropriate initial pharmacological intervention?