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
Most virulent variety of shigellosis is caused by - 
Select and explain. 
A. S. Dysenteriae, B. S. Sonnei, C. S. Flexneri, D. S. Boydii
A: S. Dysenteriae
Answer: A: S. Dysenteriae 
Explanation: None