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 36 years male presented with complaint of productive cough and fever for last 2 months. He has undergone kidney transplantation 2 years back. His sputum examination revealed a gram positive filamentous bacteria that showed acid fastness with modified Ziehl-Neelsen staining (1% H2SO4). The most likely etiological agent is ? \nSelect and explain. \nA. Actinomyces israelii, B. Nocardia asteroides, C. Blastomyces dermatitidis, D. Cryptosporidium parvum', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]