<prompt> The single most common cause of pyrexia of unknown origin isMycobacterium tuberculosisSalmonella typhiBrucella speciesSalmonella paratyphi A  <response>: 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 