A 24-year-old man from Delhi presents to the psychiatry outpatient department with a 6-month history of hearing voices that comment on his actions and converse with each other. He reports that these voices are "real" and originate from outside his head. He also believes that his thoughts are being broadcast on the radio and that neighbours are conspiring against him. His family reports a gradual decline in academic performance, poor hygiene, and social withdrawal over the past year. On mental status examination, he is alert and oriented but shows psychomotor retardation and restricted affect. There is no history of substance abuse or medical illness. What is the most likely diagnosis?