Answer: Vitamin D

For modern readers in developed nations, rickets is a childhood disease they may vaguely recall having learned about in a textbook years ago, but typically have never encountered themselves. For those in the developing world, however, rickets is still a serious affliction.

Rickets is a condition in which a child’s growing bones do not effectively mineralize due to insufficient or defective metabolizing of vitamin D, phosphorous, or calcium–with the leading cause of rickets being the lack of vitamin D. As a result of the deficiency, children grow with bowed legs, spinal and cranial deformities, and other developmental abnormalities as a result of poor bone development and fusion.

Although almost exclusively a problem in developing nations where periods of famine and undernourishment lead to a decreased intake of dietary vitamin D, there has also been a rise of cases in developed countries where poor dietary choices coupled with excessive use of sunblock (or little exposure to natural sunlight) lead to extreme vitamin D deficiencies and cases of rickets in populations that should otherwise be effectively immune.