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
Reactive ahritis is usually caused by: 
Select and explain. 
A. Shigella flexneri, B. Shigella boydii, C. Shigella shiga, D. Shigella dysenteriae
A: Shigella flexneri
Answer: A: Shigella flexneri 
Explanation: 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