Hydronephrosis is defined as the dilation of renal pelvis and calyces along with the strophic changes in the surrounding parenchyma. This condition results due to the obstruction in the urinary system that leads to retention of the urine. The buildup of urine causes increase in the pressure that leads to hydronephrosis. Hydronephrosis can be congenital or acquired. Congenital conditions includes kinked ureter, atresia of urethra. Acquired causes includes pregnancy, stones, clots, prostrate hyperplasia, urinary tract tumours and inflammatory lesions like prostatitis, urethritis and retroperitoneal fibrosis. Picture credit: Robbins and Cotran Pathologic Basis of disease