A 22-year-old man presents to the clinic complaining of low back pain and stiffness. The discomfort started several months ago as a mild dull ache in his back. The pain is more intense now, and it involves his back and hips. The symptoms seem worse at night and prevent him from sleeping. On physical examination, there is paravertebral muscle tenderness and limited flexion of the lumbar spine. Chest expansion as measured with a tape measure at the nipple line is also reduced. Figure below shows an x-ray of the lumbar spine. Which of the following is the most likely diagnosis?
High-Yield Explanation
This man's symptoms are characteristic of AS, and it occurs in l%-6% of adults inheriting human lymphocyte antigen B27 (HLA-B27). However, the prevalence in B27-positive relatives of patients with AS is up to 30%. Men are 3 times more likely to be affected. The distribution of the joints (back pain) and no hand or wrist involvement rule out RA, and osteoarthritis would be unusual in a man his age.