A 9-year-old boy with a 18-month history of recurrent inattentive episodes (3–8 daily) triggered by fatigue and mental effort is evaluated. Episodes last 15–30 seconds with gradual onset, include lip-smacking and hand fumbling, and are followed by mild post-event confusion. Between episodes, cognition and neurologic examination are normal. Video-EEG during a captured spell shows generalized spike-and-wave discharges at 3.5–4 Hz with irregular morphology and gradual onset/offset. The field maximum is marked **B** on the diagram, showing bifrontal anterior predominance. MRI brain is normal. Based on the **bifrontal anterior-predominant field maximum** and the clinical phenotype, which of the following is the most appropriate first-line antiepileptic drug?