Review Question - QID 217156

A 65-year-old man presents to the emergency department with low back pain. The pain started 6 hours ago when he was gardening and carrying a heavy bag. The pain is rated as 10/10 in severity and radiates down the posterior aspect of the right thigh. He reports no fevers or chills. His only medical problem is hypertension for which he takes amlodipine. The patient is a non-smoker, uses alcohol infrequently, and has never used illicit drugs. His temperature is 99.6°F (37.6°C), blood pressure is 140/70 mmHg, pulse is 90/min, and respirations are 20/min. Physical examination reveals 3/5 strength to hip extension, knee flexion and extension, and plantar flexion bilaterally. Sensation to pinprick is diminished over the posterolateral legs and lateral aspects of both feet. Ankle and knee reflexes are absent bilaterally. The patient’s underwear is wet and a bladder scan reveals 800 mL of urine. Which of the following is the most likely underlying diagnosis?