A 29-year-old man notes burning pain on urination with a urethral discharge that has persisted for 3 days. A sample of the exudate is positive by ELISA for Chlamydia trachomatis. The man has increasing stiffness of the knees and ankles and lower back pain 3 weeks later. A radiograph of the lumbar spine shows narrowing with sclerosis of the sacroiliac joints. One month later, he develops painful erythema of the glans penis, and the conjunctivae are red. A follow-up examination shows a slightly irregular heart rate and a murmur suggestive of aortic regurgitation. The back pain continues off and on for 5 more months. Which of the following test results is most likely to be positive in this man?