Acute pyelonephritis, photomicrograph
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Microscopically, the tubules of the medulla contain pus. If the infection continues unchecked, the release of substances from the PMNs will lead to destruction of the basement membrane of the tubular system. This destruction of the extracellular matrix would mean that the tubules would be unable to regenerate. Fibrosis would also ensue, causing the kidney to contract and scar.