Unverricht-Lundborg Disease (EPM1 / Baltic Myoclonus) - Progressive Myoclonic Epilepsy EEG

A 14-year-old boy of Finnish ancestry presents with a 2-year history of progressive action-induced myoclonus triggered by touch, sound, and flickering light. He has had three generalized tonic-clonic seizures upon awakening. Examination reveals prominent myoclonus, cerebellar dysarthria, ataxic gait, and mild cognitive decline (MoCA 24/30). Brain MRI shows mild cerebellar atrophy. The EEG pattern marked **C** in the diagram is observed, along with giant somatosensory evoked potentials. Genetic testing confirms a homozygous CSTB gene repeat expansion on chromosome 21q22.3. Which of the following is the MOST APPROPRIATE first-line antiepileptic drug for this patient?