A 22-year-old man reports that he hears someone knocking on his door at night, but every time he answers the door, no one is there. He thinks he hears his neighbors outside talking, even when he knows they are not home. He says he knows it’s his “imagination," but he feels “compelled” to answer the door, “no matter what time of day,” even if he is sleeping. He says he has had similar episodes “in the past,” but this has been going on for at least 6 months, and it has become problematic. What is the most appropriate diagnosis?

Choice A reason: The patient demonstrates obsessions (intrusive thoughts about knocking) and compulsions (compelled to answer the door) that are time-consuming and distressing, consistent with OCD. Choice B reason: Schizophrenia would involve persistent psychotic symptoms such as hallucinations, delusions, or disorganized thinking, without insight; this patient recognizes the “imagination” aspect. Choice C reason: Social anxiety disorder involves fear of social situations and judgment, not compulsions or intrusive thoughts as described here. Choice D reason: Delusional disorder involves fixed, false beliefs; the patient has insight that his experiences are irrational, making this diagnosis unlikely.