Hydronephrosis is the distention of the renal pelvis and calyces by urine , due to some obstruction along the course of the urinary passage. The obstruction may be in the ureter, the bladder, or the urethra. The term hydronephrosis is often used to include an accompanying hydroureter which develops when the obstruction is below the uretero-pelvic junction. The enlargement of the pelvis and calyces causes atrophy of the adjacent parenchyma. The amount of kidney destruction may vary from merely a flattening of the papillae to complete replacement, leaving but a thin shell of inactive cortex. When there is a superimposed infection and the renal pelvis fills with pus, the condition is called pyonephrosis.