Delusional disorder

A 42-year-old man comes to the clinic because his wife believes that he has been "losing his mind" for the past two months. He works as a lawyer and recently was named employee of the month. He believes that the Earth will be hit by an comet within the next year and has begun to stockpile water and cans of food in his basement. He admits that he has no evidence that this will occur, but simply "knows" that it is going to happen. He exercises regularly and gets 8 hours of sleep each night. Family history is significant for a brother with schizophrenia. He takes no medications. His temperature is 37.0°C (98.6°F), pulse is 68/min, respirations are 11/min, and blood pressure is 132/78 mm Hg. Physical examination shows a well-appearing male in no apparent distress. Laboratory studies are unremarkable. Which of the following is the most likely diagnosis?