Which of the following structures is included in the integumentary system?

The integumentary system is the body's external covering, which primarily includes the skin and its associated structures. The dermis is a critical component of this system; it is the thick layer of tissue just below the epidermis that contains collagen and elastin fibers, blood vessels, hair follicles, and various glands. It plays vital roles in sensation, temperature regulation, and providing structural support to the skin.

Understanding this helps to clarify the roles of other structures mentioned. Cardiac muscle is part of the muscular system, not the integumentary system. The liver and kidneys are critical organs in the digestive and urinary systems, respectively, and are not included in the integumentary system. Therefore, the dermis stands out as the correct answer, as it is an essential component specifically involved in the protection and function of the skin.