A 32-year-old man presents with a 6-month history of insidious low back pain and morning stiffness lasting 45 minutes. He reports alternating buttock pain and notes that his symptoms improve with exercise but worsen with rest. On examination, he has reduced lumbar spine mobility. An AP pelvis X-ray is obtained. The structure marked **B** in the diagram shows bilateral sclerosis and early fusion. Which of the following is the most likely diagnosis?