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: Unknown
Which is seen more commonly in rheumatoid arthritis than in ankylosing spondylitis? 
Select and explain. 
A. An HLA-B27 haplotype, B. Sacroiliitis, C. Increased incidence in men, D. Autoantibody against IgG
D: Autoantibody against IgG
Answer: D: Autoantibody against IgG 
Explanation: An autoantibody against IgG (usually IgM) describes the rheumatoid factor (RF), which is more likely to be seen in rheumatoid arthritis (RA) than in ankylosing spondylitis (AS). Ankylosing spondylitis (AS) is a sero­negative (RF negative) spondyloarthropathy that is either seen alone or in association with other disorders such as Reiter syndrome, psoriatic spondylitis, intesti­nal infections, and inflammatory bowel disease. AS most commonly involves young men between 15 and 30 years of age who are HLA-B27 positive (95% of cases). Approximately 20% of patients who are HLA­B27 positive will develop AS or one of the other variants upon exposure to the environmental factors listed above. Characteristic features include an insidi­ous onset of sacroiliitis, persistence for more than 3 months, and morning stiffness in the lower back that is relieved with exercise, as well as a diminished anterior flexion of the spine, which can be identified with the Schoeber test. The vertebral column is eventually involved with subsequent fusion of the vertebral col­umn and production of the "bamboo spine." Compli­cations include (1) restricted chest movement with development of a restrictive type of lung disease; (2) iridocyclitis, with a potential for visual loss; and (3) aortic valve incompetence with regurgitation in a small percentage of cases. Indomethacin is the treatment of choice.