Review Question - QID 213887

A 2-day-old boy is brought to the emergency department by his parents after he started vomiting numerous times within 1 hour. His parents say that he was born at home and appeared to be healthy at birth; however, he has become increasingly lethargic over the course of the past 24 hours. They became extremely concerned after he started vomiting copiously both with and without feeding. Physical exam reveals a distressed appearing infant who is hyperventilating, and laboratory tests demonstrate orotic acid in the urine. Which of the following metabolites is a substrate of the enzyme that is most likely defective in this patient?