Day 113 MCAT Practice Question

The skin can be divided into several layers from top to bottom: the

epidermis, the dermis, and the hypodermis. The epidermis primarily functions as a

physical barrier via keratinocyte cells that provide mechanical protection, but also

contains melanocytes, which are responsible for the skin’s coloring and pigmentation,

as well as the dendritic cells that serve an important role in immune function

throughout the skin. Underneath the epidermis is the dermis which contains blood

vessels, nerves, and connective tissue to create the “framework” of the skin upon

which the epidermis rests. Underneath the dermis is the hypodermis layer, which is a

fatty layer.

Answer C is correct because the skin functions to increase, rather than to reduce,

the formation of fat-soluble vitamins in the body, particularly vitamin D. 25-hydroxy

vitamin D (calcidiol) is the form of vitamin D created when the skin is exposed to

UV light. 25-hydroxy vitamin D then must undergo hydroxylation in the kidneys to

become active 1,25-dihydroxy vitamin D (calcitriol). Note that the other fat-soluble

vitamins (vitamins A, E, and K) primarily come from dietary sources.