<s>[INST] context: The patient's young age, occurrence of pain at night, negativity of rheumatoid factor, and especially, bilateral involvement of sacroiliac joints are consistent with ankylosing spondylitis. This is one of the seronegative spondyloarthropathies, characterized by onset before 40 years of age, absence of circulating autoantibodies, frequent association with HLA-B27 histocompatibility antigen, and common involvement of the spinal column. Ankylosing spondylitis should be suspected in any young person complaining of chronic lower back pain and confirmed by radiographs or CT scans of sacroiliac joints. The disease usually progresses to involve the whole vertebral column, producing ankylosis and respiratory failure secondary to restrictive lung disease. Uveitis and aortic insufficiency are additional manifestations.
Degenerative joint disease would be exceptional at such a young age, unless predisposing conditions were present. Degenerative joint disease is not associated with systemic signs and symptoms. Radiographs of affected joints show narrowed interarticular spaces, osteophytes, and increased density of subchondral bone. Sacroiliac joints are not involved. [/INST] question: A 17-year-old boy presents with chronic low back pain and stiffness for the past 8 months. He denies any GIT or genital infections. His temp is 98.6 F. Lab investigation shows normal ESR and negative RA factor. X-ray films of the vertebral column show flattening of the lumbar curve and subchondral bone erosion involving the sacroiliac joints. Most likely diagnosis? | options: A: Ankylosing spondylitis B: Degenerative join disease C: Reiter syndrome D: Seronegative rheumatoid arthritis | answer: A </s>