Pyelonephritis is a bacterial infection of the renal parenchyma and renal pelvis commonly seen in young females. It is most commonly associated with ascending spread of urinary tract infection. In cases of hematogenous spread of infection, renal abscess formation is more likely. E. coli is the most commonly found organism.

Evidence of obstruction in the setting of pyelonephritis urgently requires percutaneous nephrostomy. 

Renal vein thrombosis secondary to pyelonephritis is very rare and is usually a late complication secondary to sepsis.

Emphysematous pyelonephritis is a complicated form of acute pyelonephritis, characterized by gas bubbles within the collection system.