The single most common cause of pyrexia of unknown origin is: (A) Mycobacterium tuberculosis, (B) Salmonella typhi, (C) Brucella species, (D) Salmonella paratyphi A
Answer is A. 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