A woman believes she is being watched by the police. What is the most likely diagnosis?

The most likely diagnosis in this scenario is delusional disorder. This condition is characterized by the presence of one or more delusions that persist for at least one month. The woman's belief that she is being watched by the police suggests a fixed false belief, which is a hallmark of delusional disorder. In this context, the woman's delusion does not stem from a broader spectrum of psychotic symptoms, such as disorganized thinking, hallucinations, or negative symptoms, which are more commonly associated with disorders like schizophrenia.

In contrast, while schizophrenia also involves delusional beliefs, it typically includes additional symptoms such as hallucinations, disorganized speech, and functional impairment. Schizoaffective disorder combines features of schizophrenia with mood disorder symptoms, which does not apply in this case. Brief psychotic disorder encompasses sudden onset of psychotic symptoms (delusions, hallucinations, disorganized speech) that last more than a day but less than a month and are typically followed by a return to premorbid functioning, which again does not fit the scenario described. Delusional disorder, with its focused and specific false beliefs without the broader impairment seen in schizophrenia, is the most appropriate diagnosis for the case presented.