A 19-year-old man has a chronic papulosquamous skin disorder involving his knees and elbows. He now has dull low back pain and morning stiffness that lasts a few hours. On examination, forward flexion at the lumbar spine is reduced but neck movements are normal. There is stress tenderness of both sacroiliac joints. Which of the following is the most likely diagnosis?

This patient has a sacroiliac form of arthritis seen in patients with psoriasis. Patients with psoriasis can develop five different patterns of musculoskeletal symptoms. These include arthritis of the DIP joints; asymmetric oligoarthritis; symmetric polyarthritis similar to RA; spine and sacroiliac type; and arthritis mutilans, a highly destructive form. Most patients with psoriatic arthritis also have nail involvement. Only about a quarter actually develop a progressive, destructive disease. Uric acid may be elevated because of high tissue turnover but is not part of the pathogenesis of joint disease.