This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
[{'content': "A Bone marrow transplant recipient patient developed chest infection. On HRCT 'Tree in bud appearance' is seen. Most likely causative agent is - \nSelect and explain. \nA. Klebsiella, B. Pneumocystis, C. TB, D. RSV", 'role': 'user'}, {'content': "Answer: B \nExplanation: The 'tree-in-bud' sign is visible in high-resolution computed tomography (HRCT) and it is characterized by small centrilobular nodules of soft-tissue attenuation, connected to multiple branching linear structures of a similar caliber, originating from a single stalk.this is the main feature of pneumocystis jerovicii Ref Harrison20th edition pg 1079.", 'role': 'assistant'}]