A 64-year-old woman comes to the physician with a 10-day history of progressive difficulty walking and poor balance. Two days ago, she fell at home but did not sustain any injuries. Her chronic back pain is worse than usual and is especially bothersome at night She has no headaches, trauma, bowel or bladder incontinence, or loss of consciousness. The patient's past medical history is significant for type 2 diabetes mellitus, hypertension, transient ischemic attack, and breast cancer status post lumpectomy followed by chemoradiation 2 years ago. Her blood pressure is 162/78 mm Hg and pulse is 73fmin. Physical examination shows an unsteady gait, bilateral leg weakness, increased deep-tendon reflexes, and decreased pinprick sensation in both lower extremities up to the umbilicus. There is percussion tenderness over the lower thoracic spine. Digital rectal examination shows good sphincter tone. Mental status, memory, and cognition are intact