Movement Disorders USMLE Step 1 Practice Question

A 62-year-old woman with no significant past medical history presents to neurology with a 18-month history of progressive gait ataxia, lower extremity incoordination, and dysarthria. She denies alcohol use, recent infections, or toxic exposures. Her son was diagnosed with a similar condition at age 55. On examination, she has nystagmus, dysmetria on finger-to-nose testing bilaterally, and a wide-based gait. Vital signs are stable. Brain MRI shows selective cerebellar vermis and hemisphere atrophy with normal brainstem and cerebral white matter. Genetic testing reveals a CAG repeat expansion of 42 in the SCA1 gene. Which of the following is the most likely diagnosis?