Question

Identify the steps in the formation of vitamin \(\text{D}_{3}\):

A. Vitamin \(\text{D}_{4}\) \(\rightarrow\) 25-Hydroxycholecalciferol

B. Provitamin \(\text{D}_{3}\) \(\xrightarrow{\text{Sunlight}}\) Vitamin \(\text{D}_{3}\)

C. 7-Dehydrocholesterol \(\xrightarrow{\text{Sunlight}}\) Provitamin D

D. 25-Hydroxycholecalciferol \(\rightarrow\) Vitamin \(\text{D}_{3}\)

Choose the correct answer from the options given below:

• A. B and C only \
• B. A and D only \
• C. A and B only \
• D. D and C only

Hint:

Sunlight splits 7-dehydrocholesterol to form previtamin D, which then spontaneously rearranges into Vitamin $\text{D}_{3}$ inside the skin layers.

Answer 1

The Correct Option is A

Step 1: Concept Vitamin $\text{D}_{3}$ (cholecalciferol) can be synthesized photochemically in the skin from cholesterol derivatives when exposed to ultraviolet (UVB) radiation from sunlight.

Step 2: Meaning The pathway begins with a precursor molecule in the epidermal layers of the skin that undergoes a photochemical rearrangement when exposed to sunlight.

Step 3: Analysis Let us review the statements detailing the pathway reactions: * Statement A is incorrect: Vitamin $\text{D}_{4}$ is a distinct form of the vitamin found in mushrooms, not an intermediate in human $\text{D}_{3}$ synthesis. * Statement C is correct: The synthesis pathway begins with 7-dehydrocholesterol in the skin, which absorbs solar UVB light to break its B-ring and form provitamin D (previtamin $\text{D}_{3}$). * Statement B is correct: Once formed, provitamin $\text{D}_{3}$ undergoes a spontaneous, temperature-dependent isomerization over several hours to transform into stable Vitamin $\text{D}_{3}$ (cholecalciferol). * Statement D is incorrect: 25-hydroxycholecalciferol is formed downstream in the liver via hydroxylation of Vitamin $\text{D}_{3}$, rather than serving as its precursor.

Step 4: Conclusion Statements B and C accurately describe the early photochemical steps of Vitamin $\text{D}_{3}$ formation. This matches option A. Final Answer: (A)