MRCP2-3053

A 46-year-old man is admitted to the hospital after experiencing a tonic-clonic seizure. He has a medical history of epilepsy, gastro-oesophageal reflux, and latent TB. His current medications include sodium valproate, omeprazole, isoniazid, and carbamazepine, which was started 4 weeks ago due to poor seizure control. Additionally, he regularly consumes moderate-large amounts of grapefruit juice. Following his recovery and a Glasgow coma scale score of 15/15, a CT head scan was performed, which showed no abnormalities. What substance is responsible for causing his seizures?