A 72-year-old man with Parkinson disease has developed distressing visual experiences over the past month. His wife reports that he sets places at the table for visitors she cannot see, although he remains aware that they are unreal. Anticholinergic treatment has been withdrawn and his dopamine agonist reduced without improvement; his neutrophil count is 4.8 × 10^9/L (1.5–7.5), and he can attend regular blood tests. Which one of the following is the most appropriate antipsychotic treatment?