A 50-year-old man with Parkinson's disease develops psychotic symptoms (visual hallucinations). Which antipsychotic is MOST appropriate, and why?

• A Haloperidol; strong D2 blockade
• B Pimavanserin; 5-HT2A inverse agonist without D2 blockade ✓
• C Risperidone; balanced D2/5-HT2A blockade
• D Clozapine; low D2 affinity but not recommended in PD

Explanation

Parkinson's psychosis requires treatment that does not worsen motor symptoms by blocking dopamine receptors. Pimavanserin is an FDA-approved selective serotonin 5-HT2A receptor inverse agonist/antagonist with no dopaminergic activity—it treats psychosis without D2 blockade and therefore does not worsen Parkinson's motor features. Haloperidol and risperidone (high D2 affinity) are contraindicated as they severely worsen parkinsonism. Clozapine (very low D2 affinity, high 5-HT2A affinity) is a traditional alternative for PD psychosis but requires agranulocytosis monitoring; pimavanserin is now the preferred first-line choice for PD psychosis specifically.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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