What is the difference between multicystic kidneys and polycystic kidney disease?

Maybe no difference: Some cysts in the kidneys may be normal as people get older, and may not have anything to do with family history. Polycystic kidney disease is a genetically transmitted/caused (altho as many as 20% of patients have no family history) and in the long run frequently causes kidney failure. In polycystic kidney disease, other organs (blood vessels, liver, heart valves) may be affected also.

Aquired versus genet: Multicystic kidney dysplasia is a congenital abnormality of the connective tissue usually of one kidney which is a mass of cysts and fibrous tissue. That kidney has little or no function and can be detected with pre natal ultrasound. The disorder is not genetic. Polycystic kidney disease is a genetic disorder resulting in gradual cyst growth in both kidneys with gradual loss of kidney function.

Multi vs poly kidney: Multicystic kidney is usually a developmental anomaly of the kidney, usually unilateral and is due to the renal tissue not connecting to the developing urogenital tissue. Usually the kidney does not function, and presents during birth or early childhood. Polycystic kidneys are inherited and usually are functional but eventually will cause progressive renal failure depending on the severity.