Which term best describes a fluid-filled sac on the skin?

The term "vesicle" specifically refers to a small, fluid-filled sac that is typically located on the skin. Vesicles can form due to various conditions, such as infections, allergic reactions, or skin disorders, and they are characterized by their clear fluid inside, which differentiates them from other skin lesions.

A nodule refers to a solid, raised lump that is typically bigger than a vesicle and is usually deeper in the skin than superficial fluid-filled sacs. Macules are flat, discolored spots on the skin without elevation or fluid. An ulcer is a more complex condition where there is a loss of skin integrity, leading to deeper tissue involvement and an open sore appearance.

Understanding these definitions is crucial as it helps in accurately identifying and describing skin conditions, which is vital for diagnosis and treatment.