The description of the question stem is wrong, the correct one is cystic dilatation of the upper part of the common bile duct, meaning that the upper part of the common bile duct is widened, and 23×28 mm is the extent of the lesion. The main reasons include physiological causes and pathologic causes. 1. Physiological causes: the normal internal diameter of the common bile duct should be 6-8mm, more than 1cm suggests widening of the common bile duct. Physiologic widening is mainly seen in the elderly or patients after cholecystectomy. Elderly people can have dilatation of the common bile duct due to the reduction of biliary fiber bundles and weakening of elasticity; patients after cholecystectomy can also have compensatory widening of the common bile duct due to the loss of storage of the gallbladder. 2. Pathologic causes: mainly include congenital factors and acquired pathological changes. Congenital factors mainly refer to choledocholithiasis, that is, congenital choledochal cysts, the cause of which is unknown, and the main manifestation is cystic or spindle-shaped dilatation of the choledochal duct. Acquired pathologic changes are commonly seen in choledochal stones, cholangiocarcinoma, pancreatic cancer, etc., which are caused by poor bile drainage ducts or primary bile duct changes. Sometimes the cause cannot be accurately determined by the test results alone, and it is necessary to combine the characteristics of the medical history and other test results to make an accurate judgment, and it is recommended to clarify the cause of the disease under the guidance of the doctor.