A 27-year-old man presents to the clinic for evaluation of low back pain and stiffness. The discomfort started many months ago, but recently he has noticed more severe stiffness at night and new symptoms of hip pain. The pain improves in the morning after doing some "stretching" exercises. On physical examination, there is paravertebral muscle and sacroiliac joint tenderness on palpation with limited forward flexion of the lumbar spine. A 2/6 diastolic murmur is also heard at the left sternal border radiating to the apex. Plain x-rays of the spine are suggestive of sacroiliitis. Which of the following is the most likely diagnosis for the diastolic murmur?