Reactive arthritis

A 20-year-old man presents to campus health because he has had a painful, swollen right knee. His knee swelling began three weeks ago. He also complains of itchy, red eyes and a left-sided lower back ache which began two weeks ago. He denies any trauma. His medical history includes treatment for a chlamydia infection six weeks ago. Family history is significant for paternal-sided ulcerative colitis. Examination shows bilateral conjunctival injection. The patient also walks with a limp, and has a large effusion of his right knee. Which of the following is the most likely diagnosis?