rickets or rachitis (rəkĪˈtĭs) [key], bone disease caused by a deficiency of vitamin D or calcium. Essential in regulating calcium and phosphorus absorption by the body, vitamin D can be formed in the skin by ultraviolet rays contained in sunlight; it can also be consumed in such foods as fish oils, eggs, and butter. Since calcium and phosphorus are essential for proper development and hardening of bones, the disease manifests itself in children as softening of bones, abnormal bone growth, and enlargement of cartilage at the ends of long bones. Where bones must support weight, such as the legs and pelvis, the skeleton is likely to become bent or deformed. The result is often knock-knees, bowlegs, and deformities of the chest and pelvis. In temperate climates or the tropics, vitamin-D deficiency usually results from poor diet rather than from lack of exposure to ultraviolet rays of sunlight. Rickets is no longer common in developed countries because milk is readily obtained and is usually fortified with vitamin D; infants commonly receive vitamin D as a supplement. Treatment of rickets is largely preventive, i.e., by early recognition and by including adequate amounts of vitamin D and calcium in the diet.