This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology and Rheumatology, Subject name: Medicine
A 29-year-old man complains of back pain for several months. It takes him over 2 hours to limber up in the morning. His x-ray is shown in Figure given below. Which of the following is the most likely diagnosis? 
Select and explain. 
A. RA, B. spondylolisthesis, C. osteomalacia, D. ankylosing spondylitis
D: ankylosing spondylitis
Answer: D: ankylosing spondylitis 
Explanation: The x-ray reveals typical evidence of sacroiliitis, with widening of the joints, sclerosis, and evasions. Similar findings can be seen in psoriatic and enteropathic spondyloarthropathy.