A 21-year-old man exhibits social withdrawal, worsened academic performance, and flattened affect for three months. What is the most likely diagnosis?

The presentation of a 21-year-old man with social withdrawal, worsened academic performance, and flattened affect for three months is most consistent with a diagnosis of schizophreniform disorder. This condition is characterized by symptoms similar to schizophrenia, including delusions, hallucinations, disorganized speech, or negative symptoms, that last for at least one month but less than six months. The key aspect of this case is the duration of symptoms, which falls within the three-month timeframe essential for a diagnosis of schizophreniform disorder.

While schizophrenia requires a longer duration of symptoms—at least six months—and schizoaffective disorder involves concurrent mood episodes along with schizophrenia symptoms, the current symptomatology in this case does not meet these criteria. Major depressive disorder, while it can cause social withdrawal and affect flattening, typically does not present with the specific set of symptoms suggesting psychosis unless there are clear psychotic features, which are not indicated here. Therefore, the symptoms and duration point toward schizophreniform disorder as the most fitting diagnosis.