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: Microbiology
[{'content': 'Most virulent variety of shigellosis is caused by - \nSelect and explain. \nA. S. Dysenteriae, B. S. Sonnei, C. S. Flexneri, D. S. Boydii', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]