Image of the Week: Why does this patient's back pain remit with exercise?

A 30-year-old male presents with a 1-year history of worsening lower back pain and alternating buttock pain. He mentions that his symptoms are typically worse first thing in the morning, but they seem to remit later in the day with NSAIDs and exercise. Physical examination reveals swelling of the proximal interphalangeal joints of several digits, restricted lumbar movement, and tenderness ...