Review Question - QID 214328

A 61-year-old man with hypertension, human immunodeficiency virus infection (CD4+ count: 120 cells/µL), and alcohol abuse disorder presents to the emergency department after a fall at home. He denies head trauma and loss of consciousness. For the past 6 months, he has had multiple falls due to "losing my balance." He has consumed 3 beers per day for 25 years and denies any smoking or illicit drug history. He has been sexually active with multiple women since his early 20s and rarely uses condoms. His temperature is 99.0°F (37.2°C), blood pressure is 170/90 mmHg, pulse is 60/min, and respirations are 15/min. On physical exam, his pupils accommodate but do not constrict to direct or indirect light. His lungs are clear to auscultation bilaterally, and an early diastolic decrescendo murmur is best appreciated with the patient leaning forward. On neurologic exam, he is alert and oriented to person, place and time. He has 5/5 strength in all 4 extremities with a lack of vibratory sensation in his bilateral lower extremities. He has a wide-based gait and a positive Romberg sign. The patient is sent for MRI of the brain and spine. Which of the following is the most likely location of pathology?