This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Musculoskeletal Radiology, Subject name: Radiology
Bilateral symmetrical Sacroilitis is hallmark of : 
Select and explain. 
A. Ankylosing spondylitis, B. Juvenile Rheumatoid ahritis, C. Nail Patella syndrome, D. Osteitis condensans ilii
A: Ankylosing spondylitis
Answer: A: Ankylosing spondylitis 
Explanation: Diagnostic radiological features of Ankylosing spondylitis are: Bilateral symmetrical Sacroilitis (earliest and most specific feature) Bamboo-spine (knobby spine, poker back or universal syndesmophytosis) is a characteristic feature of it. Syndesmophytes - ossifications of the outer fibers of annulus fibrosis. Enthesitis (earliest pathological change) Uncommon involvement of small joints of hand and feet.