Hydronephrosis is a condition where the kidneys become swollen due to a blockage that prevents urine from exiting properly. It is typically caused by kidney stones, tumors, or narrowing of the ureters. Common symptoms include flank pain, nausea, frequent urination, and fever with infection. Treatment aims to drain urine from the kidneys to relieve pressure and remove the underlying cause of blockage, often through procedures to insert stents or bypass the obstruction site. Without treatment, hydronephrosis can lead to permanent kidney damage or failure.