What is a small blister or sac containing clear fluid called?

A small blister or sac containing clear fluid is known as a vesicle. Vesicles typically appear on the skin as small, fluid-filled elevations that can be caused by various conditions, including allergic reactions or viral infections like herpes. They are distinct in that they hold serous fluid, which is clear and can vary in size.

In contrast, a pustule is a similar elevation but contains pus rather than clear fluid, indicating an infection or inflammation. A macule is a flat, discolored area of skin that does not contain any raised elements, making it different from a blister. A comedo refers to a clogged pore that can result in acne, and it does not involve the characteristics of a blister or sac. Therefore, identifying a vesicle as the correct term emphasizes the specific nature of what a small blister contains and its relevance in dermatology.