Movement Disorders USMLE Step 1 Practice Question

A 45-year-old man presents to neurology clinic with an 8-year history of progressive bilateral hand tremor that interferes with writing and holding a coffee cup. The tremor is absent when his hands are resting in his lap but becomes prominent with purposeful movement and sustained posture. He reports that consuming alcohol at social events transiently reduces the tremor for several hours. His father had similar tremor beginning in his 40s, and his paternal grandfather required tremor surgery in his 60s. On neurologic examination, strength is 5/5 throughout, tone is normal, gait is steady, and there is no bradykinesia or rigidity. Cognition is intact. Brain MRI is unremarkable. Thyroid-stimulating hormone is normal. Which of the following is the most likely diagnosis?