Review Question - QID 216501

A 26-year-old man presents to his primary care physician with a 6-month history of increasing lower back pain. He first felt the pain while lifting boxes at work but thought that he had just strained a muscle. The pain appears to be worse in the mornings and after rest. Exercise and physical activity appear to temporarily make the pain better. He has taken acetaminophen and ibuprofen for the pain. He has no significant medical history and takes no medications. His temperature is 98.6°F (37.0°C), blood pressure is 115/70 mmHg, pulse is 70/min, and respirations are 12/min. On physical exam, he is found to have limited flexion and extension of his spine as well as tenderness to palpation over the insertion of his patellar tendons bilaterally. Results from laboratory tests are shown below:

Serum:
Erythrocyte sedimentation rate (ESR): 61 mm/hr
C-reactive protein (CRP): 36 mg/L (normal <10 mg/L)

Which of the following is the most accurate test for this patient's condition?