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: Medicine
Which of the organisms most commonly causes reactive ahritis? 
Select and explain. 
A. Ureaplasma urealyticum, B. Group A beta hemolytic streptococci, C. Borrelia burggorferi, D. Chlamydia
D: Chlamydia
Answer: D: Chlamydia 
Explanation: Answer is D (Chlamydia): Amongst the options provided Chlamydia is the most commonly implicated agent in Reactive ahritis. `Chlamydia Trachomatis is by far the most common cause of urethritis and of reactive ahritis following urethritis.