A 22-year-old man developed severe pain in the lower back, which radiates to heel bones, the inner surface of the thighs, there is a rise in temperature to 38.2°C. For three weeks he felt immobility in the spine, pain in the ileosacral joints. X-ray revealed: expansion of the slit of the ileosacral junction, sclerosis spinal ligaments. What disease should you think about?