Movement Disorders USMLE Step 1 Practice Question

A 45-year-old woman presents to neurology with a 5-year history of progressive involuntary twisting movements of the neck with rightward deviation, accompanied by elevation of the right shoulder and inward curling of the right hand. She reports these movements worsen with emotional stress and fatigue but improve with relaxation and when she concentrates on other tasks. She denies tremor at rest, cognitive decline, fever, or recent head trauma. Vital signs are stable. On examination, she has normal muscle strength (5/5 throughout) and normal deep tendon reflexes. Electromyography shows simultaneous contraction of agonist and antagonist muscle groups (co-contraction pattern). Brain MRI with iron-sensitive sequences is completely normal, with no basal ganglia abnormalities. Which of the following is the most likely diagnosis?