Review Question - QID 216775

A 51-year-old man is brought to the emergency department by ambulance after he is found unresponsive on a sidewalk at night. He is not alert and oriented enough to provide a history, and no identification is found in his belongings. His temperature is 97.5°F (36.4°C), blood pressure is 129/81 mmHg, pulse is 79/min, respirations are 16/min, and oxygen saturation is 99% on room air. During the physical exam, the patient is asked to keep both wrists in an extended position with the fingers outstretched. During this maneuver, he is found to have a tremor with irregular dropping of either hand followed by a jerk back to the extended position. Which of the following conditions is most likely responsible for this finding?