HTR2A and Parkinson disease: This section highlights three key adjunctive agents: pimavanserin, a selective 5-HT2A inverse agonist approved for PD psychosis (PDP) [103-105]; rivastigmine, a cholinesterase inhibitor used to treat cognitive impairment and gait instability in PD dementia [106-108]; and istradefylline, an adenosine A2A receptor antagonist used to reduce OFF episodes in patients with motor fluctuations [109].