A Patient with Partial Seizures Was on Phenytoin and Having

A patient with partial seizures was on phenytoin and having frequent breakthrough seizures. You switch her to valproic acid and she becomes lethargic with ataxia. The most likely cause:

A) The phenytoin and valproic acid had a synergistic effect
B) Valproic acid adverse effects were taking effect
C) The phenytoin was displaced from plasma proteins and reached toxic levels
D) Valproic acid catalyzed conversion of phenytoin to fosphenytoin
E) None of the above