A 60-year-old man with hypeension presents to the emergency depament complaining of veigo, nausea, vomiting and postural instability. His veigo began suddenly 2 hours ago and has been constant.Upon examination, the man cannot walk due to severe instability. The head impulse test is negative. There is left beating nystagmus on left lateral gaze and right beating nystagmus on right lateral gaze. Skew detion is present with quick, veical movements as each eye is uncovered. What is the most likely diagnosis?