Light micrograph of a human kidney in acute pyelonephritis. Pyogenic (pus-forming) kidney infections are usually caused by bacterial spread from lower urinary tract infections, particularly if there is obstruction in urinary drainage. Such obstructions may arise in benign prostatic hyperplasia or through pressure from the fetus in pregnancy. In this section a glomerulus appears at bottom centre. To the left of the glomerulus, the tissue has been invaded by dark-staining neutrophils (white blood cells), an early stage of abscess formation. On the right of the image is a dense, purple-staining clump of bacteria. BASIC HISTOPATHOLOGY FIG. 14.9