This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immune system, Subject name: Medicine
Which of the following inflammatory condition has 90% association with HLA-B27? 
Select and explain. 
A. Ankylosing spondylitis, B. Psoriasis, C. Reiters, D. Rheumatoid ahritis
A: Ankylosing spondylitis
Answer: A: Ankylosing spondylitis 
Explanation: Ankylosing spondylitis (AS) is defined by the presence of sacroiliitis on X-ray and other structural changes on spine X-rays, which may eventually progress to bony fusion of the spine. There is a male-to-female ratio of about 3 : 1. In Europe, more than 90% of those affected are HLA-B27-positive (Caucasian HLA-B27 population prevalence is 9%). The overall prevalence of AS is below 0.5% in most populations. Over 75% of patients are able to remain in employment and enjoy a good quality of life. Even if severe ankylosis develops, functional limitation may not be marked, as long as the spine is fused in an erect posture. Ref Davidsons 23e p1030