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
Reactive arthritis is usually caused by- 
Select and explain. 
A. Shigella flexineri, B. Shigella boydli, C. Shigella shiga, D. Shigella dysentriae
A: Shigella flexineri
Answer: A: Shigella flexineri 
Explanation: None