Most common organism associated with reactive ahritis is:

Chlamydia [Ref: Harrison 21st edition Reactive ahritis is a systemic disorder of unknown etiology that is defined by the development of conjunctivitis, urethritis, ahritis, and mucocutaneous lesions following an episode of infection elsewhere in the body. In 1916, Hans Reiter described the triad of nongonococcal urethritis, conjunctivitis, and ahritis in a young German officer with bloody dysentery.The classic triad of the disease, namely urethritis, ahritis, and conjunctivitis, is present in only one third of the patients. Reactive ahritis is frequently associated with the human leukocyte antigen B27 (1-ILA-B27) haplotype. The etiology of reactive ahritis remains unceain. The most accepted theory about the pathophysiology of reactive ahritis involves initial activation by a microbial antigen, followed by an autoimmune reaction that involves the skin, eyes, and joints. Two forms are recognized: a sexually transmitted form and a dysenteric form. Gastrointestinal infections with Shigella, Salmonella, and Campylobacter species and other microorganisms, and genitourinary infections especially with Chlamydia trachomatis have been found to trigger reactive ahritis. Young children tend to have the post dysenteric form, whereas adolescents and young men are most likely to acquire reactive ahritis after they have urethritis.