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
Not a radiological feature of Ankylosing spondylitis 
Select and explain. 
A. Dagger sign, B. Trolley track sign, C. Romanus sign, D. Fish mouth vertebrae
D: Fish mouth vertebrae
Answer: D: Fish mouth vertebrae 
Explanation: None