An otherwise healthy 17-year-old girl presents with a 4-month history of episodes of déjà vu or unpleasant olfactory aura, followed by a period of staring, confusion, and unresponsiveness that lasts 30 to 90 seconds. She then develops headache, nausea, and extreme fatigue. More recently, the episodes are associated with depersonalization, a sinking feeling, and periods of “blacking out.” She has mild short-term memory deficits on examination. MRI scan of the brain is normal. Which of the following medications is most likely to improve her symptoms?