A patient is brought with 6 month history of odd behaviour. There is history of a family member having disappeared some years back. He seems to be talking to himself and sometimes muttering to himself loudly. The likely diagnosis is:

Patient in this question is not having typical delusions and hallucinations of schizophrenia but the history of odd behaviour for 6 months makes schizophrenia the most likely diagnosis. The other options in the question does not have this type of clinical manifestations. For the diagnosis of schizophrenia, two or more of the following symptoms such as delusions, hallucinations, disorganized speech, grossly disorganized behavior and negative symptoms should be present for 6 months. In conversion disorder, patients develops symptoms of motor or sensory dysfunction which are not intentionally produced. These symptoms can occur after a traumatic event. Delusional disorders are psychoses in which the predominant symptoms are persistent, nonbizarre delusions with minimal impairment of daily functioning. In this there is little occupational and intellectual impairment whereas social and marital functioning are markedly affected. Ref: CURRENT Medical Diagnosis & Treatment 2014 chapter 25, CURRENT Diagnosis & Treatment: Psychiatry, 2e chapter 16.