The incidence of pyelonephritis is roughly 12–13 cases annually per 10,000 population in women receiving outpatient treatment and 3–4 requiring admission. In men, 2–3 per 10,000 are treated as outpatients and 1–2/10,000 require admission. Young women are most often affected, probably reflecting sexual activity in that age group. Infants and the elderly are also at increased risk, reflecting anatomical changes and hormonal status. Xanthogranulomatous pyelonephritis is most common in middle-aged women. It can present somewhat differently in children, in whom it may be mistaken for Wilms' tumor.
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