A 22-year-old man presents to the clinic complaining of low back pain and stiffness. The discomfort started several months ago as a mild dull ache in his back. The pain is more intense now, and it involves his back and hips. The symptoms seem worse at night and prevent him from sleeping. On physical examination, there is paravertebral muscle tenderness and limited flexion of the lumbar spine. Chest expansion as measured with a tape measure at the nipple line is also reduced. Figure below shows an x-ray of the lumbar spine. Which of the following is the most likely diagnosis?