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': 'The primary site of multiplication of rickettsial organisms is in the - \nSelect and explain. \nA. Parenchymal cells of the liver, B. Endothelial cells of small vessels, C. Media of arteries, D. Adventitia of all blood vessels', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]