Which of the following describes a closed, abnormally developed sac that contains fluid or pus?

A closed, abnormally developed sac that contains fluid or pus is defined as a cyst. Cysts are encapsulated structures that can form in various tissues of the body and may arise from a variety of causes such as infections, blockages in glands, or inflammatory conditions. They can become inflamed or infected, leading to the accumulation of pus, which is fluid that contains dead cells, bacteria, and tissue debris.

In contrast, contact dermatitis is an inflammatory skin condition resulting from direct contact with an irritant or allergen, and it does not involve the formation of a sac. A fissure refers to a linear crack or break in the skin that does not fit the description of a sac. Dermatitis is a broad term for inflammation of the skin, encompassing various conditions that cause redness, swelling, and irritation, but it does not specifically refer to a closed sac containing fluid or pus. Therefore, the definition of a cyst aligns perfectly with the characteristics described in the question.