You are answering a USMLE-style medical question. Question: A 3-day-old boy is brought to the emergency department because of a 4-hour history of somnolence, poor feeding, and one episode of vomiting. He is exclusively breastfed. His serum glucose concentration is 88 mg/dL and his serum ammonia concentration is 850 μmol/L (N<90). Urinalysis shows an increased orotic acid to creatinine ratio. Urinary ketones are absent. This patient is most likely deficient in an enzyme that catalyzes which of the following reactions? Options: A) Ornithine and carbamoyl phosphate to citrulline B) Orotic acid to uridine monophosphate C) Pyruvate to acetyl-coenzyme A D) Glutamate and acetyl-coenzyme A to N-acetylglutamate Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.