Final answer:
The patient's presentation of having a singular delusional belief regarding being poisoned with no other psychotic symptoms or functional impairment for more than a month strongly suggests a diagnosis of Delusional disorder.
Step-by-step explanation:
The most likely diagnosis for a patient who presents with a delusion about being poisoned for 5 months, without hallucinations or other psychotic symptoms, and with no major impact on daily functioning or mood symptoms, is Delusional disorder. This condition is characterized by the presence of one or more non-bizarre delusions lasting for at least one month. On the other hand, schizophrenia involves a range of symptoms including hallucinations, delusions, and negative symptoms, which typically impair functioning. Schizoaffective disorder includes symptoms of schizophrenia along with mood disorder symptoms, which this patient does not have. Brief psychotic disorder is characterized by a short duration of symptoms and typically resolves within a month. Lastly, dementia involves cognitive decline and is not characterized solely by delusions, and Parkinson's disease-like symptoms are not associated with delusions as a primary feature.