This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: AIIMS 2019, Subject name: Orthopaedics
Most specific for diagnosing Ankylosing spondyliti? 
Select and explain. 
A. HLA B27, B. B/l sactoilitis, C. Lumbar movement, D. ESR
B: B/l sactoilitis
Answer: B: B/l sactoilitis 
Explanation: According to Modified New York criteria, the diagnosis of definite AS requires the following: established sacroiliitis on radiographs (unilateral or bilateral) and at least one of the following clinical criteria: (1) low back pain and stiffness for more than 3 months improving with activity, (2) reduced chest expansion (3) limited lumbar movements. HLA B27 is present in > 90% cases of ankylosing spondylitis but it is not specific for AS.