Review Question - QID 214409

A 65-year-old man presents to the emergency department with lower back pain. He states that it started 2 days ago and has been gradually worsening. He has a past medical history of diabetes and obesity and a 40 pack-year smoking history. His initial vitals are unremarkable, and his physical exam on presentation reveals normal strength, sensation, reflexes, and rectal tone. Despite treatment with acetaminophen, ibuprofen, lidocaine patches, and morphine, the patient complains of a sudden worsening of his back pain while in the ED and is requesting more IV pain medications. His temperature is 98.3°F (36.8°C), blood pressure is 87/45 mmHg, pulse is 158/min, respirations are 22/min, and oxygen saturation is 99% on room air. Which of the following is the most likely diagnosis?