A 42-year-old man who has been employed by a company for the past 2 years believes that,h is fellow employees have hatched a conspiracy to get him fired, he believes that they have tapped his phone and that they follow him home. He frequently checks his home for cameras as he believes that fellow employees are keeping an eye on him. He denies having auditory hallucinations and there are no other symptoms. Which of the following is the most appropriate diagnosis for this man?