FCA1-0567

A 39-year old man came to the Out-Patient department for symptoms of gastroesophageal reflux disease. Medical history revealed he is on anti-epileptic medication Phenytoin. His plasma phenytoin levels are maintained between 10-12 mcg/mL (Therapeutic range: 10-20 mcg/mL). He is given a H2 antagonist receptor agent (Cimetidine) for his GERD symptoms. Upon follow-up, his plasma phenytoin levels increased to 38 mcg/mL. Regarding metabolism and elimination, which of the following best explains the pharmacokinetics of phenytoin at higher plasma levels?