A 72-year-old male with Parkinson's disease managed on carbidopa-levodopa presents with his wife, who reports he has been seeing small animals in the house that are not there and has become increasingly suspicious that neighbors are entering the home at night. His UPDRS motor scores are well-controlled on his current regimen. The PMHNP recognizes that standard antipsychotics could worsen his parkinsonism. Which of the following best describes the mechanism that makes pimavanserin uniquely appropriate for this patient?