A 4-yrs-old boy is brought to the physician by his parents for episodes of waking in the middle of the night screaming. The parents state that when they get to the boy's room during one of these episodes, they find him in his bed, thrashing wildly, his eyes wide open. He pushes them away when they try to comfort him. After 2 min, the boy suddenly falls asleep, and the next day he has no memory of the episode. Which of the following medication is used to treat this disorder?