A 3-year-old boy presents with recurrent episodes of lethargy, vomiting, and developmental delay. Plasma ammonia is markedly elevated (>200 µmol/L). Genetic testing reveals a deficiency in carbamoyl phosphate synthetase I (CPS I). His 5-year-old sister has similar biochemical findings but remains asymptomatic with normal development. Which feature best distinguishes symptomatic from asymptomatic urea cycle defects in this family?