A 68-year-old woman comes to the clinic complaining of gait ataxia, dysarthria, and dysphagia that have been present for the past 2 years. Upon examination, the patient exhibits downbeating nystagmus and slurred speech. Both upper limbs show past pointing, and the patient has a wide-based ataxic gait. Reflexes and sensation are normal, and there is no wasting or fasciculations. The plantar response is flexor bilaterally. What is the most likely diagnosis?