A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and vomiting. On examination, he is hypotonic with a weak cry. Laboratory investigations reveal: blood ammonia 380 µmol/L (normal <50), pH 7.32, HCO₃⁻ 18 mEq/L, and normal liver function tests. Plasma amino acid analysis shows elevated glutamine and alanine. Urine orotic acid is markedly elevated. Which enzyme deficiency is most likely responsible for this presentation?