Pyelonephrosis is an enlargement of the renal pelvis and calyces accompanied by atrophy of the renal tissue due to urinary tract obstruction. Urinary tract obstruction can occur anywhere in the urinary tract, either unilaterally or bilaterally. The degree of obstruction can be complete or incomplete and can cause hydronephrosis after a certain period of time. If there is a narrowing or obstruction of any part of the urinary tract and normal neuromuscular dysfunction, the passage of urine can become obstructed, resulting in urinary flow obstruction. The pressure in the part above the obstruction gradually increases due to poor urine discharge, and the lumen expands, eventually leading to hydronephrosis, expansion, thinning of the renal parenchyma, and decreased renal function. Differential diagnosis of the phenomenon of hydronephrosis: 1. Polycystic kidney The age of onset is 40-60 years old, and more than half of the patients have hypertension in combination. Cystic masses can be palpated on one or both sides of the upper abdomen. IVU shows elongation or deformation of the renal pelvis and calyces under pressure without dilatation. Ultrasonography and radionuclide kidney scan showed that both kidneys were enlarged and there were multiple round cysts in the kidney area. CT examination showed that both kidneys were enlarged and most cystic masses with smooth edges and different sizes were seen in the renal parenchyma. 2.Simple renal cysts Cystic masses can often be palpated when the volume increases. Ultrasonography shows a single round dark area with neat edge in the renal area, IVU shows compression, deformation and displacement of the renal pelvis and calyces without fluid, CT examination shows a round thin-walled, well-defined low-density mass with enhanced density in the renal parenchyma but no enhancement of the mass. 3. perinephric cyst A cystic mass with unclear boundaries may appear in the lumbar region, with poor mobility and insignificant fluctuation; however, there is often a history of trauma. ivu shows that the kidney is shrunken and displaced, but the renal pelvis and calyces are normal in shape without dilatation. Ultrasonography shows a transmissive dark area around the kidney. 4. adrenal cyst A large cystic mass can be found in the lumbar region. annular calcification can be seen on X-ray; IVU shows downward displacement of the kidney and displacement of the renal axis by pressure, without deformation and dilatation of the renal pelvis and calyces. Retroperitoneal insufflation angiography, ultrasonography, and CT examination all show images of cystic masses in the adrenal region. 5. mesenteric cyst A cystic mass with clear margins can be palpated in the abdomen. However, the mass is superficial and moves to the left and right; there are symptoms of intestinal obstruction; there are signs of compression on barium meal X-ray examination of the gastrointestinal tract. 6, pancreatic cyst A cystic mass with unclear edges can be palpated in the left upper abdomen. However, it is often accompanied by a history of abdominal trauma or acute pancreatitis; mostly seen in adults; no urinary manifestations; positive urine glucose test; signs of compression on barium X-ray of the gastrointestinal tract. 7. Hepatic cysts Cystic masses can be palpated in the right upper abdomen or under the glabella. However, the cyst is superficially located, easily palpable, and the pressure pain is obvious; it is not accompanied by urinary symptoms; ultrasonography and radionuclide liver scan show signs of cyst. 8.Horseshoe kidney A homogeneous and substantial mass is palpated in the umbilical region of the abdomen. In case of hydronephrosis, an irregular cystic mass can be palpated, but IVU shows that the renal axis is inverted octagonal in shape, and the shadow of the isthmus connecting the two kidneys can be seen in the middle; the calyces on both sides are lower and closer to the midline, and the calyces extend medially. 9. Mild hydronephrosis of the kidney and ureter is often present during normal pregnancy In addition to the compression of the ureter by the pregnant uterus, it is due to the relaxation of the renal ureteral muscles caused by the secretion of progesterone during pregnancy. This is a physiological change that almost always occurs on the right side due to the anatomical relationship.