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': 'Portal of entry of poliovirus in mainly - \nSelect and explain. \nA. Gastrointestinal tract, B. Nasal mucosa, C. Lung, D. Skin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]