Which of the following is a precursor to vitamin D in humans?

• A. Alpha-tocopherol
• B. 7-dehydrocholesterol
• C. Phylloquinone
• D. Retinol

Answer: (B)

Verified step by step guidance

1. Understand that a precursor is a substance from which another is formed, especially by metabolic reaction. In the context of vitamins, a precursor is a compound that the body can convert into the active vitamin form.
2. Recall that vitamin D in humans is synthesized in the skin upon exposure to UVB radiation. This synthesis starts from a specific precursor molecule present in the skin.
3. Identify the known precursors of vitamin D: 7-dehydrocholesterol is the compound in the skin that, when exposed to UVB light, converts to previtamin D3, which then becomes vitamin D3 (cholecalciferol).
4. Review the other options: Alpha-tocopherol is a form of vitamin E, Phylloquinone is vitamin K1, and Retinol is vitamin A. None of these serve as precursors to vitamin D.
5. Conclude that among the given options, 7-dehydrocholesterol is the correct precursor to vitamin D in humans.