The causes of hydronephrosis can be divided into congenital, acquired, and external factors. Among these, acquired causes are the most common, primarily referring to obstructions in the urinary tract, such as stones that block the ureter, leading to hydronephrosis, inflammation, or ischemic damage. Scarring of the ureter can also result in hydronephrosis. Tumors or polyps in the renal pelvis and ureter may also cause obstructions leading to fluid accumulation. External causes mainly include diseases of retroperitoneal organs, such as abscesses, bleeding, tumors in the retroperitoneum, and pelvic tumors, like rectal cancer. Congenital causes mainly involve intrinsic narrowing of the ureter, ectopic vessels causing compression, and high insertion of the ureter, which are less common.