This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
Earliest investigation for diagnosis of Ankylosing spondylitis - 
Select and explain. 
A. MRI STIR sequence, B. Bone scan, C. CT scan, D. X- ray
A: MRI STIR sequence
Answer: A: MRI STIR sequence 
Explanation: Dynamic MRI with fat saturation, either short tau inversion recovery (STIR) sequence or T1-weighted images with contrast enhancement, is highly sensitive and specific for identifying early intraarticular inflammation, cartilage changes, and underlying bone marrow edema in sacroiliitis.
Magnetic resonance imaging allows for visualization of acute sacroiliitis, spondylitis, and can also detect acute inflammation of the entheses, bone, and synovium. The ability to detect early inflammation and accurately visualize cartilaginous and enthesal lesions makes magnetic resonance imaging a useful assessment tool in the spondyloarthropathies.