<prompt> The single most common cause of pyrexia of unknown origin is -Mycobacterium tuberculosisSalmonella typhiBrucella sppSalmonella paratyphi A  <response>: Pyrexia of unknown origin is definedvas a temperature persistently above 38 degree celsius for more than 3 weeks,without diagnosis despite initial investigations during 3 days of inpatient care or after more than two outpatient visits. Specific organisms associated with PUP are tuberculosis extrapulmonary - most common cause, HIV-1 infection,other viral infections cytomegalovirus,EBV, fungal infections Aspergillus spp.,candida spp., dimorphic fungi, infections with fastidious organismsBaonella spp., Tropheryma whipplei. Reference Harrison20th edition pg 1232 