A 4-year-old boy presents with recurrent seizures since age 2 years. His parents report sudden falls without warning (drop attacks), brief episodes of stiffening especially at night, and brief lapses of awareness. EEG shows generalized slow spike-and-wave discharges at 1-2 Hz with abnormal background activity. He has progressive cognitive decline. The clinical and electrographic findings are consistent with the condition marked **D** in the diagram. Which of the following management strategies is MOST appropriate as a first-line agent specifically approved for reducing drop attacks and atypical absences in this syndrome?