Ascending Infection Drives Acute Pyelonephritis; VUR and Obstruction Fuel Spread Acute pyelonephritis is a rapidly developing bacterial inflammation of the kidney’s renal pelvis, an upper urinary tract infection. Most cases arise from ascending infection, as bacteria colonize the urethra and bladder and travel up the ureter; risk rises with female sex, sexual intercourse, indwelling catheters, diabetes mellitus, and urinary tract obstruction. Vesicoureteral reflux lets urine flow backward when the vesicoureteral orifice fails, strongly promoting spread, while obstruction causes urinary stasis that favors bacterial adherence. Ascending disease is usually due to bowel flora such as E. coli, Proteus, and Enterobacter. Less commonly, hematogenous seeding from septicemia, bacteremia, or infective endocarditis infects the kidney, typically with Staphylococcus species or E. coli.
Neutrophilic Interstitial Inflammation Yields WBC Casts and Systemic Clues; Early Antibiotics Avert Complications Infection is most often unilateral; bacteria adhere to renal tubular epithelium, triggering chemokine-driven neutrophil influx into the interstitium while sparing glomeruli and vessels. Neutrophils appear in urine, sometimes forming white blood cell casts, and peripheral blood may show leukocytosis. Fever, chills, nausea, vomiting, and flank pain at the costovertebral angle signal systemic involvement that distinguishes it from lower UTI. Treatment is typically antibiotics and hydration, with potential for renal abscess; recurrent infections from anatomic problems can progress to chronic pyelonephritis and papillary necrosis, impairing kidney function.