What term describes a closed sac that develops abnormally in the body?

The term that describes a closed sac that develops abnormally in the body is a cyst. Cysts are encapsulated structures filled with liquid, semi-solid, or gaseous material, and they can occur in various tissues throughout the body. They often form as a result of obstruction in ducts, infection, or other conditions that lead to the abnormal growth of tissue.

In contrast, a blister is a raised area of skin filled with clear fluid, typically caused by friction, heat, or certain infections. Urticaria, also known as hives, refers to a skin reaction characterized by itchy welts or bumps, usually due to an allergic response. Xanthomas are fatty deposits that form under the skin, often associated with lipid metabolism disorders, rather than being fluid-filled sacs. Therefore, the defining characteristics of a cyst make it the correct choice for describing a closed sac that develops abnormally in the body.