Final answer:
A burn extending into the fascia and/or muscle is known as a fourth-degree burn, which is severe and may require medical procedures like debridement and skin grafting.
Step-by-step explanation:
A burn that extends into the fascia and/or muscle is classified as a fourth-degree burn. This type of burn is characterized by the full thickness damage to the skin, as well as harm to the underlying structures like muscle and bone. These burns are often not as painful immediately because the nerve endings are destroyed, but they are extremely severe and require immediate medical attention.
Treatment often involves procedures such as excision (debridement), potential amputation in severe cases, and skin grafting.