An 80-year-old male with a 10-year history of hypertension presents with sudden increased sleepiness, mouth deviation, and weakness of the right upper and lower limbs. Examination reveals left ptosis, incomplete upward, downward, and adduction movements of the left eye, right central facial and tongue palsy, grade 0 muscle strength in the right upper and lower limbs, bilateral Babinski sign, and right-sided facial and hemibody hypoesthesia. Brain MRI shows long T1 and T2 signals in the left midbrain and bilateral thalamus. The most likely diagnosis is: