Anterior Uveitis & Iridocyclitis — Practice

A 35-year-old man presents with recurrent episodes of redness and mild pain in the right eye over the past year, each episode lasting 2–3 weeks. He also gives a history of low back pain that is worse in the morning and improves with activity. Slit-lamp examination reveals fine keratic precipitates, flare, and cells in the anterior chamber with posterior synechiae. The most likely underlying systemic association is: