A 41-year-old man presents with stiffness and swelling in both knees, hips, and ankles. His height is 5 ft 10 in and weight is 170 lb (BMI, 24.5). He has been waking up in the middle of the night with back pain, though it improves in the morning, and he notes that he has hardly been able to work on his computer because of stiffness in the hands. He has no notable medical history but reports that his maternal aunt had type 1 diabetes. Dactylitis of the fingers and nail lesions are seen.