Movement Disorders USMLE Step 1 Practice Question

A 35-year-old man presents with a 5-year history of progressive dystonia that began in his left foot. Over the past year, dystonia has developed in his left leg, and more recently in his right arm and neck. He works as a software engineer and reports chronic occupational stress. He denies medication use, drug exposure, or family history of movement disorders. Neurologic examination reveals sustained muscle contractions causing abnormal postures and repetitive twisting movements; there is no weakness, sensory loss, hyperreflexia, or pyramidal signs. Brain MRI, serum ceruloplasmin, 24-hour urine copper, and serum prolactin are all normal. Levodopa challenge test shows no response. Which of the following is the most likely diagnosis?