A 26-year-old male presents with intermittent low back pain for 5 years and left knee joint swelling/pain for 1 year. Physical exam: left knee joint swelling/tenderness, negative patellar ballottement test, positive bilateral '4' sign, occiput-to-wall distance 15cm. Investigations: HLA-B27 positive, sacroiliac CT shows bony sclerosis, joint space narrowing, irregular articular surfaces with erosions and cystic changes. Scout image shows 'bamboo spine' appearance. The most likely diagnosis is: