Final answer:
Sunlight triggers melanocyte-stimulating hormones to produce melanin, protecting the skin from UV damage and aiding in the prevention of folic acid breakdown.
Step-by-step explanation:
The trigger for melanocyte-stimulating hormones (MSH) to produce melanin is sunlight (A). When the skin is exposed to UV rays from sunlight or a tanning salon, it stimulates keratinocytes to secrete chemicals that activate melanocytes. Melanocytes then increase the production of melanin, which accumulates in keratinocytes leading to skin darkening, or a tan. This melanin production plays a crucial role in protecting the DNA of epidermal cells from UV ray damage and preventing the breakdown of folic acid, a vital nutrient. However, an excess of melanin can also affect vitamin D production, which is critical for calcium absorption.