What characterizes a blister?

A blister is characterized by a collection of fluid that accumulates between the top layer of skin (epidermis) and the underlying layer (dermis). This fluid buildup typically occurs in response to friction, heat, or irritation, creating a protective bubble over the affected area. The presence of this fluid serves as a cushion, allowing the damaged skin underneath to heal without further irritation.

Other options describe different types of injuries. A deep puncture wound involves a penetration through the skin that can damage underlying tissues, but it does not create a fluid-filled sac. An avulsion refers to a situation where layers of skin are torn away, which is quite different from the formation of a blister. Lastly, a pressing injury that exposes underlying tissues is typically not associated with fluid accumulation, but rather with damage caused by direct pressure or trauma. Thus, the distinguishing feature of a blister is specifically the presence of the fluid collection underneath the skin.