This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Rheumatoid Ahritis, Subject name: Medicine
[{'content': 'Reactive ahritis is usually caused by: \nSelect and explain. \nA. Shigella flexneri, B. Shigella boydii, C. Shigella shiga, D. Shigella dysenteriae', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Organisms that have been associated with Reiter Ahritis include the following: C trachomatis (L2b serotype) Ureaplasma urealyticum Neisseria gonorrhoeae Shigella flexneri Salmonella enterica serovars Typhimurium Mycoplasma pneumoniae Mycobacterium tuberculosis Yersinia enterocolitica and pseudotuberculosis Campylobacter jejuni Clostridium difficile Beta-hemolytic (example, group A) and viridans streptococci', 'role': 'assistant'}]