A 75-year-old diabetic man with Parkinson disease complained of worsening of his tremor and rigidity in his arms and legs. His Parkinson disease had been responding well to a treatment with levodopa/carbidopa and amantadine, and his diabetes had been controlled by glyburide and metformin. Recently, metoclopram ide was prescribed to manage diabetic gastroparesis. Which of the following drugs could have triggered the worsening of symptoms reported by the patient?