A 7-year-old girl presents with brief staring spells occurring multiple times daily for 3 months. During events she stops mid-sentence, stares blankly with subtle eyelid flutter, lasting 8–12 seconds with immediate recovery and no postictal confusion. Bedside hyperventilation reliably reproduces a typical spell. Video-EEG shows the pattern marked **A** in the diagram—a generalized symmetric 3 Hz spike-and-wave discharge with abrupt onset and offset on a normal background, activated by hyperventilation. MRI brain and metabolic panel are normal. Based on this classic EEG finding, which of the following is the first-line antiepileptic drug for this condition?