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': 'A 20 year old man presented with hemorrhagic colitis. The stool sample grew Escherichia coli in pure culture. The following serotype of E. coli is likely to be the causative agent - \nSelect and explain. \nA. 0 157 : H7, B. 0 159 : H7, C. 0 107 : H7, D. 0 55', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., 0157 : 117", 'role': 'assistant'}]