Rickets, nutritional disorder characterized by skeletal deformities. Rickets is caused by a decreased concentration of the mineral hydroxyapatite in bones and cartilage due to low levels of calcium and phosphorus in the blood. Vitamin D is essential for the maintenance of normal calcium and phosphorus levels. Classic rickets, a deficiency disease of children characterized by improper development or hardening of bones, is due to lack of sufficient vitamin D in the diet, or to insufficient ultraviolet radiation from direct sunlight, a lack that prevents conversion of the element 7-dehydrocholesterol in the skin to vitamin D. The type of skeletal deformities depends largely on the child's age when the vitamin-D deficiency occurs. A child who has not yet learned to walk develops vertebral curvatures; a walking child develops leg curvatures. "Rickets," Microsoft® Encarta® Online Encyclopedia 2000 http://encarta.msn.com © 1997-2000 Microsoft Corporation. All rights reserved.