What type of skin lesion is a cyst?

A cyst is classified as a fluid-filled lesion that forms due to the accumulation of fluid, pus, or other substances within a closed sac or cavity in the skin or tissue. This characteristic defines it distinctly from other types of skin lesions.

Fluid-filled cysts can vary in size and may be palpable under the skin, often appearing as a bump. They are generally surrounded by a fibrous capsule, which helps to differentiate them from other lesions that do not have this enclosed structure.

In contrast, other listed types of lesions do not fit the description of a cyst. For example, a crater refers to a hollowed-out area often associated with ulcers or other skin conditions, while redness typically indicates inflammation rather than a specific type of lesion. An ulcer is a break in the skin or mucous membrane that typically involves a loss of tissue and may not be fluid-filled. Thus, the defining feature of a cyst being fluid-filled solidifies why this is the correct identification of the lesion type.