Lalu prasad 45 yrs old male presents to OPD with complaining about the continuous non-progresive headache from last 7 days. He believes that he has a brain tumor. for that, there is he consulting many neurologists in past even all of the investigation have been within normal limits. The patient requested for other investigation to reveal that he has a brain tumor. The psychiatric evaluation shows the disease on the background of a normal investigation. The most probable diagnosis is -