A 6-month-old boy presents with a unilateral port-wine stain in the V1 distribution and focal seizures that began at 3 months of age. MRI brain shows ipsilateral cortical atrophy with characteristic gyriform calcifications. The structure marked **D** in the diagram is responsible for the neurologic manifestations in this patient. Which of the following is the MOST appropriate FIRST-LINE antiepileptic drug for seizure management in this child?