A 45-year-old man with generalized tonic-clonic seizures has been taking phenytoin 300 mg daily for 3 years with good seizure control. Over the past 6 months, he has experienced an increase in seizure frequency despite reported medication adherence and unchanged dose. On examination, he has coarse facial features, marked gingival hyperplasia, and hirsutism. Laboratory studies show: serum total phenytoin level 18 μg/mL (therapeutic range 10-20 μg/mL), serum albumin 4.2 g/dL (normal), and normal renal and hepatic function. He reports new-onset persistent headaches and difficulty concentrating. Which of the following best explains the loss of seizure control despite a therapeutic total phenytoin level?

1. A)Phenytoin-induced folate deficiency causing increased seizure susceptibility
2. B)Development of drug-metabolizing enzyme inhibition secondary to gingival hyperplasia
3. C)Decreased serum free (unbound) phenytoin concentration due to altered protein bindingGABARITO
4. D)Nonlinear (zero-order) metabolism of phenytoin leading to unexpected kinetic changes
5. E)Phenytoin-induced hypothyroidism lowering the seizure threshold