A 3-year-old boy presents with developmental delay, seizures, and hyperammonemia (plasma ammonia 180 µmol/L). Genetic testing reveals a deficiency in carbamoyl phosphate synthetase I (CPS I). His 6-year-old sister has similar symptoms but normal CPS I activity; genetic testing shows ornithine transcarbamylase (OTC) deficiency. Which clinical feature best distinguishes OTC deficiency from CPS I deficiency?

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