A 4-day-old female neonate is under evaluation in the neonatal ICU following a
seizure-like episode. Since starting
breastfeeding, she has been irritable and feeding poorly, and she has had several episodes of vomiting. She was born at 29 weeks gestation to a 29-year-old woman following a caesarean delivery for prolonged
labor. The patient has been exclusively
breastfed since birth. Temperature is 36.4°C (97.5°F),
pulse is 145/min, and
respiratory rate is 67/min. On physical examination, the patient appears
somnolent and
lethargic.
Skin turgor is delayed and eyes appear sunken. Abdominal examination is unremarkable. Neurological examination reveals diffuse hypotonia and
somnolence without response to painful stimulation.
Sepsis workup is negative. Laboratory evaluation reveals
orotic aciduria and hyperammonemia. Serum arginine levels are within normal limits. Deficiency of which of the following enzymes is responsible for this patient’s symptoms?
