The so-called hydronephrosis is a morphological description, referring to an obstruction in the urinary system caused by various reasons. After the kidneys produce urine, it cannot be normally delivered to the bladder. This leads to fluid accumulation in the renal pelvis and calyces, which can be detected through imaging as hydronephrosis. The causes of hydronephrosis are numerous. The most common cause is acquired obstruction, primarily due to urinary system stones, scarring from injuries, or tumors in the urinary system compressing the ureter, which often results in urinary tract obstruction. In addition, some congenital causes can also lead to urinary tract obstruction. Examples include segmental dysfunction of the ureter, intrinsic stenosis of the ureter, and twisting of the ureter. However, these congenital obstruction causes are relatively rare.