A 9-year-old boy cannot go to sleep without having a parent in his room. While falling asleep, he frequently awakens to check that a parent is still there. One parent usually stays until the boy falls asleep. If he wakes up alone during the night, he starts to panic and gets up to find his parents. He also reports frequent nightmares in which he or his parents are harmed. He occasionally calls out that he saw a strange figure peering into his dark room. The parents usually wake in the morning to find the boy asleep on the floor of their room. They once tried to leave him with a relative so they could go on a vacation; however, he became so distressed in anticipation of this that they canceled their plans. Which of the following is the least likely diagnosis?