Final answer:
Ultraviolet (UV) light catalyzes the synthesis of vitamin D in the human skin by converting cholesterol into cholecalciferol, which then becomes the active form of vitamin D, essential for bone health.
Step-by-step explanation:
Vitamin D Synthesis and Ultraviolet Light
Vitamin D synthesis in the human skin occurs when it is exposed to UV radiation, specifically UV-B rays from sunlight. Cholecalciferol, a form of vitamin D3, is synthesized from a derivative of cholesterol present in the skin. The liver and kidneys play a pivotal role in converting this into calcitriol, the active form of vitamin D important for bone health and absorption of calcium and phosphorus.
Ultraviolet (UV) light is thus essential for vitamin D synthesis in the body. This synthesis is vital for preventing bone-related diseases such as rickets and osteomalacia. Given the importance of sunlight in this process, people living in regions with limited sun exposure may often rely on supplements and fortified foods to meet their vitamin D requirements.
Therefore, the correct answer to the student's question about what is needed to synthesize vitamin D, given the precursor is available, is A) Ultraviolet (UV) light.