Which of the following is NOT a layer of the skin?

The mesodermis is not a recognized layer of the skin. The skin is typically composed of three main layers: the epidermis, dermis, and hypodermis.

The epidermis is the outermost layer, providing a protective barrier and playing a key role in the skin's overall health and function. The dermis lies beneath the epidermis and contains blood vessels, nerves, hair follicles, and connective tissue—an essential layer for structural support and nutrient supply to the epidermis. The hypodermis, or subcutaneous layer, is primarily composed of fat and connective tissue, serving as insulation and cushioning for the body while also anchoring the skin to underlying structures.

Since the term "mesodermis" does not correspond to a specific layer of the skin, it stands out as the correct choice in this context, while the other options represent established anatomical components of skin structure.