Hydronephrosis is a build-up of urine inside the kidney, usually because the drainage pipe is blocked or too narrow. Pressure stretches the kidney like a water balloon and sends quiet but clear warnings. Flank pain is the headline. It feels like a deep ache just below the ribs or a sudden cramp that can shoot into the groin when the blockage shifts. Urinary changes follow. You may notice less urine, a weak stream, or the need to go more often with only small amounts coming out. Blood in the urine can appear. The color may be light pink or cola-brown,... Learn more