Which of the following is an example of a primary lesion?

A cyst is classified as a primary lesion because it represents a distinct type of skin abnormality that is formed from a collection of fluid or semi-solid material within a sac. Primary lesions are those that are directly attributable to a disease process or condition and reflect the initial reaction of the skin. Cysts can vary in size and can occur anywhere on the body, often serving as a hallmark of certain skin disorders.

In contrast, a crust is typically a secondary lesion that forms when a primary lesion, like an ulcer or a vesicle, has exuded fluids that have dried. A keloid is also a secondary lesion, characterized by an overgrowth of scar tissue resulting from a healing process. Lastly, a fissure is a form of secondary lesion that involves a crack in the skin, usually occurring after a primary lesion has developed and healed improperly or due to excessive dryness. Knowing these classifications helps in understanding how various skin conditions present and progress.