Two siblings are evaluated for hyperammonemia. The elder brother (age 8) has a history of protein intolerance and developmental delay, with plasma ammonia 250 µmol/L and markedly elevated plasma argininosuccinate (180 µmol/L, normal <7). His younger sister (age 5) has similar symptoms but with plasma ammonia 200 µmol/L and normal argininosuccinate levels (4 µmol/L). Genetic testing confirms argininosuccinate lyase deficiency in the brother and argininosuccinate synthetase deficiency in the sister. Which laboratory finding best distinguishes argininosuccinate lyase deficiency from argininosuccinate synthetase deficiency?