A patient with a delusion about being poisoned for 5 months without other psychotic symptoms is most likely diagnosed with?

The diagnosis of delusional disorder is particularly appropriate in this scenario as the patient exhibits a persistent, non-bizarre delusion—in this case, the belief that they are being poisoned. Delusional disorder is characterized by the presence of one or more delusions that persist for at least one month, with the capacity for an otherwise functioning individual to maintain their sense of reality in other domains of life. This aligns with the patient’s two main features: a well-formed delusion persisting for 5 months and the absence of other psychotic symptoms such as hallucinations or disorganized thinking.

In the context of the other potential diagnoses, dementia does not fit because it typically involves cognitive impairment and the presence of multiple cognitive deficits rather than isolated delusions. Schizophrenia would entail a broader range of symptoms impacting thought processes, emotions, and behaviors, not just a single delusion without other characteristics of the disorder. Brief psychotic disorder involves the sudden onset of psychotic symptoms that last for at least a day but less than a month, which again does not match with the sustained delusional belief found in this patient.

Therefore, delusional disorder is the most fitting diagnosis given the specific symptoms and duration presented.