Ans. is 'a' i.e., E Coli o Bacterial infections are the most common cause of UTI, with E coli being the most frequent pathogen, causing 75- 90% of UTIs. Other bacterial sources of UTI include the following:# Klebsiella species# Proteus species# Enterococcus species# Staphylococcus saprophyticus, especially among female adolescents and sexually active females# Streptococcus group B, especially among neonates# Pseudomonas aeruginosao Fungi (Candida species) may also cause UTIs, especially after instrumentation of the urinary tract. Adenovirus is a rare cause of UTI and may cause hemorrhagic cystitis.