Final answer:
Melanin production is triggered by an increase in melanocyte-stimulating hormone (MSH) in response to exposure to UV light.
Step-by-step explanation:
Melanin production is triggered by an increase in melanocyte-stimulating hormone (MSH) in response to exposure to UV light. MSH is produced by the pituitary gland and stimulates melanocytes in the skin to produce melanin. This increased melanin production results in the darkening of the skin, or a tan, as a protective response to UV ray damage.