A 27-year-old man has a history of low back pain and stiffness. After several months of mild symptoms, he notes more severe stiffness at night and hip pain. On physical examination, there is paravertebral muscle tenderness and limited flexion of the lumbar spine. X-ray of the lumbar spine shows sacroiliitis. In addition to recommending physiotherapy and exercise, which of the following is the most appropriate next step in management?