A 34-year-old man presents with back pain for the past 6 months. His pain is associated with morning stiffness, and it actually improves with exercise but not with rest. His back pain bothers him most at night. He also has pain in his left ankle. Family history is negative for rheumatologic diseases. On physical examination, there is swelling and warmth over his left Achilles tendon. He has increased spinal kyphosis and tenderness over both sacroiliac joints. Imaging of his SI joints reveals narrowing of the joint spaces and sclerosis of the endplates on the iliac side. What condition does this patient most likely have?