Rickets: A disease of infants and children that disturbs normal bone formation (ossification), leading to failure to mineralize bone. Rickets softens bone, producing osteomalacia, and permits marked bending and distortion of bones. Other features of rickets include softness of the infant's skull (craniotabes), enlargement of the front end of the ribs (creating the 'rachitic rosary'), thickening of the wrists and ankles, lateral curving of the spine (scoliosis), abnormal forward'backward curving of the spine (kyphosis and lumbar lordosis), and deforming and narrowing of the pelvis. As the child begins to walk, the weight on the soft shafts of the legs results in knock-knees or, more often, bowlegs. Until the first third of the 20th century, rickets was usually due to lack of direct exposure to sunlight or to lack of vitamin D, calcium, and phosphorus, but nutritional rickets has become relatively rare in industrialized nations. In developing countries, vitamin D'deficiency rickets continues to be a problem. Rickets in developed countries is usually now due to other causes, such as disorders that create vitamin D deficiency by interfering with the absorption of vitamin D through the intestines; diseases of the liver, kidney, or other organs that impair the normal metabolic conversion and activation of vitamin D; and conditions that disrupt the normal balance in the body between calcium and phosphorus.