A 15-year-old boy presents to the neurology clinic with a 2-year history of brief, bilateral jerking movements of his shoulders and arms occurring every morning upon awakening. He reports dropping objects and spilling his coffee during these episodes. Over the past year, he has also experienced two episodes of generalized tonic-clonic seizures, both occurring in the early morning. His parents report a family history of generalized epilepsy. EEG shows the pattern marked **A** in the diagram — generalized discharges with multiple spikes followed by slow waves at 4-6 Hz, bilaterally synchronous and maximal over frontocentral regions. Brain MRI is normal. Which of the following is the most appropriate first-line antiepileptic drug for this patient?