A 30 year old man since 2 months suspects that his wife is having an affair with his boss. He thinks his friend is also involved from abroad and is providing technological suppo. He thinks people talk negatively about him. His friends tried to convince him but he is not convinced at all. Otherwise he is normal, doesn't have any thought disorder or other inappropriate behavior. The most likely diagnosis is which of the following?

The patient in the question is suffering from persistent delusional disorder. This diagnosis can be made when a person suffers from persistent delusion involving a real life situation (like infidelity) for more than one month in the absence of any formal thought disorder or inappropriate behavior.Ref: International Statistical Classification of Diseases and Related Health problems By World Health Organization, 2nd edition page 309; DeGowin's Diagnostic Examination, by Richard F. LeBlond, 9th edition, chapter 15.