22. A 12-year-old boy with a long history of Lennox-Gastaut syndrome was admitted to the neurologic clinic for evaluation. In the past, the boy had been treated with several drugs, but the response to the antiepileptic therapy was rather poor. After a neurologic examination and an electroencephalogram, the neurologist decided to prescribe an antiepileptic drug with a broad spectrum of action, including partial, tonic-clonic, myoclonic, and atonic seizures, as well as infantile spasm and Lennox-Gastaut syndrome. Which of the following drugs was most likely prescribed?