If a patient presents with psychosis for 3 weeks which resolves completely, how is this episode classified?

The situation described in the question corresponds to Brief Psychotic Disorder, which is characterized by the sudden onset of psychotic symptoms such as delusions, hallucinations, disorganized speech, or grossly disorganized or catatonic behavior lasting at least one day but less than one month. The key detail here is that the psychotic episode lasts for three weeks and resolves completely, fitting the criteria for Brief Psychotic Disorder.

In contrast, schizophrenia requires symptoms to persist for a minimum duration of six months, not merely three weeks. Major Depressive Disorder can include psychotic features, but the defining factor is the presence of a major depressive episode, which does not align with the brief duration and complete resolution of psychotic symptoms presented. Delusional Disorder involves the presence of one or more delusions that persist for at least one month, but again, it does not fit since the psychosis resolves within a shorter timeframe, and it typically centers on specific delusional themes rather than a broad array of psychotic symptoms. Thus, the resolution of symptoms within a three-week period aligns perfectly with Brief Psychotic Disorder, making it the correct classification for this episode.