This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardio thoracic surgery  , Subject name: Surgery
[{'content': "The organism most frequently related to mediastina fibrosis is \nSelect and explain. \nA. Actinomycosis, B. Histoplasma, C. Hansen's bacillus, D. Staphylococcus", 'role': 'user'}, {'content': 'Answer: B \nExplanation: Fibrosing mediastinitis or mediastinal fibrosis is the least common,but the most severe ,late complication of histoplasmosis.It is due to an abnormal immunological response to antigens released by the soil based fungus Histoplasma capsulatum.It is characterised by invasive calcified fibrosis centered at locations of lymph nodes,which occludes major vessels & airways.', 'role': 'assistant'}]