A 35-year-old man with a 20-year history of focal-onset seizures with impaired awareness presents to the epilepsy clinic. He reports that seizures begin with an epigastric rising sensation, followed by a period of unresponsiveness with lip-smacking and chewing movements, and postictal confusion lasting several minutes. Brain MRI with epilepsy protocol shows the finding marked **A** in the diagram. He has failed trials of levetiracetam, lamotrigine, and oxcarbazepine at adequate doses. Which of the following is the most appropriate next step in management?