Q. A 2-year-old boy presents to the pediatrician due to recurrent seizures and developmental delay. The mother reports that the child has a severe intellectual disability, has not started walking, and has a musty odor. She also mentions that her son was healthy and asymptomatic until about 6 months of age. On physical examination, the patient has hypopigmented hair and skin. Laboratory studies show markedly increased levels of phenylalanine in the blood. The patient's clinical presentation suggests a disorder in which the patient lacks a factor that primarily acts on which of the following steps of arginine metabolism?