Acute pyelonephritis is an ''exudative purulent localized inflammation'' of the renal pelvis (collecting system) and kidney.
The renal parenchyma presents in the interstitium abscesses (suppurative necrosis), consisting in purulent exudate (pus): neutrophils, fibrin, cell debris and central germ colonies (hematoxylinophils).
Tubules are damaged by exudate and may contain neutrophil casts. In the early stages, glomeruli and vessels are normal.
Gross pathology often reveals pathognomonic radiations of hemorrhage and suppuration through the renal pelvis to the renal cortex.
Chronic infections can result in fibrosis and scarring.
Xanthogranulomatous pyelonephritis is a form of chronic pyelonephritis associated with granulomatous abscess formation and severe kidney destruction.
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