The most likely diagnosis is ankylosing spondylitis, which is characterized by inflammatory back pain that manifests as pain and stiffness in the spine that is worse after immobility and better with use. Symptoms are prominent in the morning (>1 hour), and patients can be symptomatic during the night. Buttock pain is common and correlates with sacroiliitis, which is typically bilateral. This patient has symptoms/signs consistent with ankylosing spondylitis, including more than 3 months of inflammatory back pain of primarily axial involvement, age of onset younger than 45 years, a positive HLA-B27, and a good response to an NSAID. The lack of sacroiliitis or other inflammatory changes on his radiographs does not rule out this diagnosis; these changes may not be evident early in the disease course and may not be seen on plain radiographs if there are no bone erosions. He fulfills the Assessment of SpondyloArthritis international Society (ASAS) classification criteria for axial spondyloarthritis because he has a positive HLA-B27 plus at least two other features of spondyloarthritis, including inflammatory back pain and a good response to NSAIDs. The ASAS classification criteria use a nomenclature that defines spondyloarthritis as axial or peripheral, and ankylosing spondylitis would be the prototype disease in the spectrum of axial spondyloarthritis. These criteria allow patients who have not yet developed radiographic sacroiliitis to be classified as having “non-radiographic” axial spondyloarthritis.

Distinguishing between inflammatory and noninflammatory joint pain is critical in evaluating patients with musculoskeletal conditions. Inflammation may be the only symptom that distinguishes ankylosing spondylitis from lumbar degenerative disk disease. Subjective manifestations of joint inflammation include morning stiffness for more than 1 hour. Lumbar degenerative disk disease is not likely in this patient because his radiographs are normal and he has inflammatory back pain.

Characteristic features of psoriatic arthritis include enthesitis, dactylitis, tenosynovitis, arthritis of the distal interphalangeal joints, asymmetric oligoarthritis, and spondylitis. The HLA-B27 antigen may be positive in patients with axial involvement. Psoriatic arthritis involving only the axial skeleton is possible in this patient but less likely because he has no evidence of psoriasis.

Reactive arthritis (formerly known as Reiter syndrome) is a postinfectious arthritis that occurs in both men and women. Infections may include urethritis or diarrhea, although patients may be asymptomatic. Arthritis, usually oligoarticular, develops several days to weeks after the infection. The HLA-B27 antigen may be positive in these patients. Reactive arthritis is also less likely as this patient has no history of a gastrointestinal or genitourinary infection preceding the onset of arthritis.