A 30-year-old man has had cramping abdominal pain and bloody diarrhea for the past 4 days. On physical examination, there is diffuse tenderness on palpation of the abdomen. Bowel sounds are present. There are no masses and no organomegaly. A stool culture is positive for Shigella flexneri. The episode resolves spontaneously within 1 week after onset. Six weeks later, he has increasingly severe lower back pain. Physical examination now shows stiffness of the lumbar spine and tenderness of the sacroiliac joints. He is treated with nonsteroidal anti-inflammatory agents. Two months later, the back pain recurs, and he complains of redness of the right eye and blurred vision. Serologic testing for which of the following is most likely to be positive in this patient?