A 5-day-old male neonate born to a healthy mother presents with lethargy, poor feeding, and a single seizure. Serum ammonia is 180 µmol/L (normal <50), blood urea nitrogen is 8 mg/dL (low), and urine orotic acid is markedly elevated. The enzyme marked **B** in the urea cycle diagram is deficient. Which of the following best explains the elevated urinary orotic acid in this patient?