Merck Manual

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Hydronephrosis: A Distended Kidney
Distended Kidney (2)
Hydronephrosis: A Distended Kidney

In hydronephrosis, the kidney is swollen (distended) because the flow of urine is obstructed by a kidney stone. Urine backs up behind the obstruction and remains in the kidney’s small tubes and central collecting area (renal pelvis).