In cases of hydronephrosis, urinary tract infections are common, and these infections primarily manifest as pyelonephritis. The presence of fluid in the kidneys indicates that urine cannot be excreted smoothly, leading to an obstruction in the urinary tract. Therefore, bacteria can easily multiply in this area because normal urine, if it can be excreted smoothly, also serves to flush the urinary system, preventing the growth of bacteria. Thus, when fluid accumulates, pyelonephritis can easily occur, with the main symptoms being back pain and fever. If cystitis is also present, the patient will experience symptoms such as frequent urination, urgent urination, and painful urination. Hydronephrosis may be caused by issues such as urinary system stones or tumors, which can also lead to clinical symptoms like back pain and hematuria.