Which of the following is considered an appendage of the skin?

Hair is classified as an appendage of the skin because it is a structure that grows from the epidermis, the outermost layer of the skin. Skin appendages also include nails and sweat and sebaceous glands. These structures all originate from the skin and contribute to its function and protection. Hair plays a crucial role in sensory perception, temperature regulation, and protection against environmental factors.

In contrast, the adrenal glands, muscles, and lymph nodes are not considered skin appendages as they are distinct organs or structures with specific functions unrelated to the skin's primary role. Adrenal glands are part of the endocrine system, secreting hormones; muscles facilitate movement and are part of the muscular system; lymph nodes are integral to the immune system, involved in filtering lymph and housing immune cells. Thus, hair is the only choice that aligns with the definition of an appendage of the skin.