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Urinary tract infection (UTI) is defined by the presence of microorganisms within the urinary tract, which is usually sterile. Since asymptomatic colonization of the urinary tract can occur, definitive diagnosis often relies upon a constellation of features that might include history and examination findings, elevated inflammatory markers, and repeat urine cultures. UTIs are typically divided into lower tract disease, where infection is localized to the bladder and urethra (cystitis and urethritis), and upper tract disease, where it extends to the ureter and kidney (pyelonephritis). Although both upper and lower tract disease may result in significant morbidity, pyelonephritis in particular is associated with renal scarring and subsequent hypertension, chronic renal disease, and preeclampsia.1,2

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UTIs are the most common serious bacterial infections affecting infants and young children. In recent decades, UTI has been increasingly recognized as an important occult cause of fever in young children. Rates of UTI vary widely with respect to age, gender, race, and other factors. Screening studies performed in emergency departments suggest an overall prevalence of UTI of up to 5% in febrile children younger than 2 years.3,4 Peak incidence of UTI occurs in the first year of life for all children, with a second peak occurring among female adolescents. After infancy, females are far more likely than males to have a UTI. A population-based European study reported a cumulative UTI incidence of 7.8% for girls by age 7 years.5 One factor influencing the relatively higher rates of UTI in male infants is circumcision status; uncircumcised males younger than a year are approximately 10 times more likely to develop UTI than their circumcised counterparts.6 In young children, race appears to be an independent risk factor for UTI. In an emergency department study, Caucasian females younger than 2 years with fever ≥39°C have a UTI prevalence of 16% compared to a 2.7% prevalence among nonwhite girls.4

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Bacterial pathogens cause the vast majority of UTIs, but viruses, fungi, and parasites can cause infection as well. UTI occurs when enteric stool pathogens or skin flora ascend through the urethra, infecting the bladder or spreading further into the upper urinary tract. The shorter urethra in females has been implicated in their predisposition to UTI. Similarly, uncircumcised infants harbor increased numbers of uropathogenic bacteria in the periurethral area.7 Bacterial invasion is the result of the interaction between bacteriologic properties such as adhesion, virulence, and motility as well as anatomic and genetic properties that influence host response.8 Some racial and genetic differences may be explained by differences in blood group antigens on the surface of uroepithelial cells, which affect bacterial adherence. An association of certain Lewis blood group phenotypes has been found in children with UTIs9 and in women with recurrent UTIs.10 Studies suggest that there may be a genetic predisposition to acute pyelonephritis caused by an inherited defect in neutrophil migration and activation.11 Rarely, in young infants, infection may be ...

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