A 3-year-old boy from North India presents with recurrent episodes of vomiting, lethargy, and developmental delay. Plasma ammonia is 180 µmol/L (normal <50). Genetic testing reveals a deficiency in carbamoyl phosphate synthetase I (CPS I). His cousin, who has ornithine transcarbamylase (OTC) deficiency, presents with similar symptoms but has a different biochemical profile. Which finding best discriminates CPS I deficiency from OTC deficiency?