A 26-year-old man presents to the office complaining of persistent back pain for the past three months. You saw the patient previously and prescribed non-steroidal anti-inflammatory drugs (NSAIDs) and rest, but he has not improved. The man states that his maternal grandfather had “back problems his whole life,” and he worries that he might have the same problem. An X-ray is performed and shows squaring of the vertebrae with longitudinal fibrous bands. What is the most likely diagnosis?