Review Question - QID 216470

A 54-year-old man presents to the clinic with 4 weeks of persistently worsening back pain. The pain is localized to 1 spot in his lower back and is worse with physical activity. His medical history is pertinent for intravenous drug use. He has no past surgical history. His temperature is 36.8°C (98.2°F), blood pressure is 118/90 mmHg, pulse is 92/min, respirations are 13/min, and oxygen saturation is 99% on room air. On physical exam, he has midline back tenderness to palpation at L2-L3. Laboratory workup reveals an erythrocyte sedimentation rate of 112 mm/h and C-reactive protein of 10 mg/dL. Which of the following is the most likely diagnosis?