A 17-year-old girl presents 2 days following a generalized seizure, her second in 6 months. Both seizures occurred between 3 AM and 6 AM while she was sleeping, in the week before her menses. Her medical history is unremarkable and she takes no medications. Results of physical and neurologic examinations are normal, and a pregnancy test done in the emergency department is negative. EEG shows activity consistent with a generalized seizure disorder. What medication is best at this time?