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
[{'content': 'Reactive arthritis is usually caused by- \nSelect and explain. \nA. Shigella flexineri, B. Shigella boydli, C. Shigella shiga, D. Shigella dysentriae', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]