This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
[{'content': 'The single most common cause of pyrexia of unknown origin is \nSelect and explain. \nA. Mycobacterium tuberculosis, B. Salmonella typhi, C. Brucella species, D. Salmonella paratyphi A', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Tuberculosis is an ancient disease, infection is common in India. So it is the most common cause of Pyrexia of Unknown origin, compared to others. Ref: Baveja textbook of microbiology; 4th edition', 'role': 'assistant'}]