A 2-month-old male neonate presents with lethargy, poor feeding, and hyperammonemia (ammonia 250 µmol/L). Plasma amino acid analysis shows elevated glutamine and alanine but low citrulline and arginine. Urine orotic acid is normal. A second patient, age 5 years, has recurrent hyperammonemic crises triggered by infection, with similar amino acid profile but markedly elevated urinary orotic acid. Which laboratory finding best distinguishes carbamoyl phosphate synthetase I (CPS I) deficiency from ornithine transcarbamylase (OTC) deficiency in these two patients?