A 20-year-old woman is brought to the emergency room by her fam- ily because they have been unable to get her to eat or drink anything for the past 2 days. The patient, although awake, is completely unresponsive both vocally and nonverbally. She actively resists any attempt to be moved. Her family reports that during the previous 7 months she became increasingly withdrawn, socially isolated, and bizarre; often speaking to people no one else could see. Which of the following is the most likely diagnosis?