A 59-year-old man is evaluated for a 10-month history of hand tremor. The tremor is more prominent when he is tired, anxious, or walking. He also reports occasional difficulty with fine-motor movements, a deterioration of his handwriting, and a softening of his voice. His gait is not magnetic, and he has had no problem with balance or falls. The patient does not have urinary incontinence or dementia. His paternal grandfather had essential tremor and his paternal aunt had Parkinson disease.
On physical examination, vital signs are normal. Facial expression is normal, and his voice is low in volume but not tremulous. With the patient at rest, a low-frequency tremor is noted in the right hand and chin. With the arms in an outstretched position, a bilateral tremor emerges after a delay of several seconds that is more prominent on the right side. Finger-to-nose testing reveals a mild bilateral tremor that does not worsen near the target. Rapid alternating movements of the right upper and lower extremities become slower and shorter in amplitude with repetition. Muscle tone is increased bilaterally, with step-wise resistance to passive movements. Gait is normal, but arm swing is decreased and a tremor emerges on the right side during ambulation.
Which of the following is the most appropriate next step in confirming the diagnosis?