A vesicle is defined as:

• A. A pustule
• B. Circumscribed, elevated lesion containing serous fluid <5 mm
• C. Vesicle containing pus
• D. Plateau-like lesion >10 mm

Answer: (B)

A vesicle is defined by its content and size: a circumscribed, elevated lesion filled with clear serous fluid and measuring less than 5 millimeters in diameter. This description fits vesicles seen in many rashes and infections where the fluid is clear, not pus. It sets vesicles apart from pustules, which contain pus, and from bullae, which are similar blisters but larger than 5 mm. Plaques are flat-topped, plateau-like lesions larger than about 1 centimeter. So the description of a small, well-defined, fluid-filled lesion under 5 mm containing serous fluid is precisely what defines a vesicle.