The photograph shows the renal papillary region of the medulla. The top of the photograph shows transitional epithelium that lines the calyceal lumen. The arrows point to collecting ducts. There is chronic inflammation involving the parenchymal interstitium. Urine reflux due to uretero-vesicle incompetence with or without superimposed infection.
Most cases are due to ascending infections from urine flow abnormalities.
Urinary outflow obstruction results in urine stasis providing an environment for bacterial proliferation. Obstruction also predisposes to vesicoureteral reflux which can enable bacteria to reach the kidney. In general, any abnormality to the flow of urine from the calyces to the urethra increases the risk for pyelonephritis. There is irregular loss of renal parenchyma with scarring. There is distortion of the associated calyces which commonly appear dilated because of the reduction in medullary and cortical tissue.