Final answer:
A burn injury that affects the entire dermis and extends into the subcutaneous tissue is known as a third-degree burn, which requires medical attention and may involve treatments such as excision and grafting.
Step-by-step explanation:
A burn injury that extends through the entire dermis and into the subcutaneous tissue is categorized as a third-degree burn. This type of burn fully extends into both the epidermis and dermis, destroying the tissue and affecting nerve endings and sensory function. Unlike a fourth-degree burn, which is even more severe and affects underlying muscle and bone, a third-degree burn does not impact the subcutaneous layer to that extent. These burns require serious medical attention and will heal slowly, as they cannot be repaired by the body alone. Treatments typically involve excision (debridement), and possibly grafting of the skin.