Final answer:
A vesicle is a fluid-filled circumscribed lesion; it's essentially an encapsulated sac just beneath the skin's upper layer. It differs from pustules or abscesses, which are pus-producing, and from flat rashes that have no fluid elevation.
Step-by-step explanation:
A vesicle is c. A fluid-filled circumscribed lesion. Vesicles are small, encapsulated sacs filled with fluid, such as clear serum or plasma, and are typically located just below the upper layers of skin. These lesions might be seen in various skin infections and conditions, such as impetigo, which causes the formation of vesicles, pustules, and sometimes larger fluid-filled bullae. Pus-filled lesions would more accurately be termed pustules or abscesses, and a vesicle differs from a cyst by having a less defined boundary.
Vesicles are different from flat rashes or discolorations of the skin, which are more superficial and do not have the elevated, fluid-filled characteristics of a vesicle. They also stand apart from pus-producing lesions, commonly found in infections such as furuncles and carbuncles, where the lesions are suppurative—in other words, they produce pus. The typical aspect of a vesicle could be compared to the fluid- or pus-filled bumps on the skin that are a result of various infections, though strictly speaking, vesicles contain fluid rather than pus.