As fluid builds up in the kidneys, they become swollen. The blockage or obstruction makes it difficult to urinate, and the more the kidneys swell, the more they press against other organs. One of the main symptoms of hydronephrosis is a pain in the flank or the side of your back, under your lower ribs. This pain may travel to the groin or stay localized, but generally means you should see a doctor.

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