Review Question - QID 214246

A 14-year-old boy reports a 2-month history of low back pain with no known trauma. The pain is worse with activity and relieved by rest, although he does report difficulty with prolonged sitting in school. He reports no history of radicular pain and denies any numbness, tingling, or weakness in his legs. On exam, he has slight tenderness to palpation over the lower back, 5 out of 5 strength in the lower extremities, and a negative straight leg raise test. The lumbar spine demonstrates full range of motion but he reports pain with back extension. Initial AP and lateral radiographs of the spine are negative. Which of the following should be ordered next to determine the patient's diagnosis?