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What is causing this patient's low back pain?

Physical examination The patient was well appearing with a typical nonantalgic gait. His vital signs were within normal limits. Spinal examination was signifi cant for mild decreased forward fl exion. Tenderness to palpation was observed diffusely across the lumbar paraspinal muscles, sparing the spinous processes, but without palpable spasms or gross deformity. Rectal tone was preserved. Lower extremities were free of overt signs of involvement, demonstrating an intact neuromuscular examination with normal sensation, straight leg raise, and deep tendon refl exes. He had benign skin, neck, lung, heart, abdomen, and genitalia examinations. Diagnostic testing and initial management Peripheral access was obtained, and he was given morphine 4 mg IV and a bolus of 500 mL 0.9% sodium chloride solution. Lumbar spine radiograph was ordered and revealed degenerative joint disease without evidence of acute osseous defects. Complete blood cell count, basic metabolic panel, and urinalysis were within normal limits, except for mild elevations in the blood urea nitrogen, 24.4 mg/dL (normal range, 8 to 23 mg/dL); creatinine, 1.28 mg/dL (normal range, 0.67 to 1.17 mg/dL); and anion gap, 14.7 mmol/L (normal range, 5 to 14 mmol/L). During the clinical course, the patient began to express signs of rapid decline, including bilateral lower extremity cyanosis, loss of sensation, and the absence of distal pulses (including with Doppler examination). In light of the marked clinical progression, an emergency IV contrast CT of the chest, abdomen, and pelvis with venous runoff to the bilateral lower extremities was promptly arranged.