A 22-year-old man develops the insidious onset of low back pain improved with exercise and worsened by rest. There is no history of diarrhea, conjunctivitis, urethritis, rash, or nail changes. On examination, the patient has loss of mobility with respect to lumbar flexion and extension. He has a kyphotic posture. Which test or group of tests would be most supportive of your suspected diagnosis?

As indicated in the question, insidious back pain occurring in a young male and improving with exercise suggests one of the spondyloarthropathies-ankylosing spondylitis, reactive arthritis (formerly termed Reiter syndrome), psoriatic arthritis, or enteropathic arthritis. In the absence of symptoms or findings to suggest one of the other conditions, laboratory tests that show a mild anemia, a positive HLA-B27 (present in 90% of patients with ankylosing spondylitis), an elevated sedimentation rate, and symmetrical sacroiliitis on x-ray would all be very supportive of the diagnosis of ankylosing spondylitis. The prognosis in ankylosing spondylitis is generally good, with only 6% dying of the disease itself. While pulmonary fibrosis and restrictive lung disease can occur, they are rarely a cause of death (cervical fracture, heart block, and amyloidosis are leading causes of death as a result of ankylosing spondylitis). The MRI described in answer a would be more suggestive of spinal tuberculosis (Pott disease) or metastatic cancer. Rheumatoid factor is negative in all the spondyloarthropathies as would be the anti-CCP test, although the C-reactive protein level could be elevated in any inflammatory condition. Joint space narrowing and osteophytes are the hallmarks of osteoarthritis which would be unusual in a young man. While Crohn disease can cause an enteropathic arthritis, which may precede the GI manifestations, this diagnosis is far less likely in this case than ankylosing spondylitis.