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: Surgery
Which of the following is not usually involved in ankylosing spondylitis? 
Select and explain. 
A. Sacroiliac joint, B. Spine, C. Ankle, D. Elbow
D: Elbow
Answer: D: Elbow 
Explanation: Usually axial skeleton is severely involved. Elbow joint is not involved. Commonest joint involved is Sacroiliac joint.