Polycystic kidney is a kind of renal cyst. Renal cysts are mainly divided into two types. One is simple renal cyst, that is, a single fluid-filled cyst appears in the kidney, and the specific cause is unknown. This kind of cyst is mostly asymptomatic, and only when it becomes very large and causes pain in the side abdomen or back will symptoms appear. Simple renal cysts may develop into malignant tumors, but this rarely happens. Another kind of cyst is a hereditary disease called polycystic kidney.