Review Question - QID 214207

A 74-year-old man presents to the emergency department with dizziness. He states that it started suddenly this morning when he woke up from sleep. It has persisted for the past 5 hours with only minor improvement in his symptoms. The patient feels off balance and has had to hold on to a wall to maintain his balance during this time frame. He has a past medical history of atrial fibrillation, hypertension, diabetes, and smoking and is currently taking metoprolol, apixaban, lisinopril, insulin, metformin, and bupropion. The patient recently recovered from an upper respiratory infection. His temperature is 99.0°F (37.2°C), blood pressure is 167/91 mmHg, pulse is 90/min, respirations are 13/min, and oxygen saturation is 96% on room air. Physical exam is notable for an elderly man with an ataxic gait. Head impulse exam reveals an absence of a corrective saccade with appropriate fixed gaze on the target. A bidirectional nystagmus is noted on cranial nerve examination. Test of skew is notable for skew deviation. The patient's strength is 5/5 in the upper and lower extremities with normal sensation. A Dix-Hallpike maneuver is performed, and the patient states that his baseline dizziness seems slightly worse during the maneuver. Which of the following is the most likely etiology of this patient's dizziness?