Congenital megacalycosis is a rare condition that is caused by a genetic mutation. It is caused by a mutation in the gene that codes for the enzyme carbonic anhydrase II (CAII). This enzyme is responsible for the production of bicarbonate, which is necessary for the proper functioning of the kidneys. Without this enzyme, the kidneys are unable to properly filter out waste products from the body, leading to an accumulation of calcium in the kidneys. This accumulation of calcium can lead to the formation of large cysts in the kidneys, which can cause a variety of symptoms.