A 7-year-old girl is brought to the pediatric clinic by her mother who reports multiple episodes of behavioral arrest lasting 10–15 seconds throughout the day. During these episodes, the child stares blankly, exhibits mild eye fluttering, and resumes normal activity immediately without confusion. The EEG shows the pattern marked **A** in the diagram — a generalized, bilaterally synchronous, symmetric discharge at 3 Hz on a normal background. Hyperventilation during EEG provokes additional bursts. Which of the following drugs is the most appropriate first-line agent for this condition?