Which term describes swelling of pus?

The term that describes swelling of pus is "pustule." A pustule is a small, inflamed, blister-like elevation on the skin that contains pus. It is often associated with infections and inflammatory conditions. The presence of pus indicates an accumulation of leukocytes, primarily neutrophils, which are responding to an infection, usually caused by bacteria.

In contrast, a vesicle is a small fluid-filled blister that does not necessarily contain pus; it might hold clear fluid or serum. Granulomas are small clusters of immune cells that form in response to persistent inflammation or infection, and they do not describe a localized collection of pus. Phlegmon refers to a diffuse inflammatory process, usually involving connective tissue, leading to the infiltration of pus and is not limited to a well-circumscribed elevation on the skin like a pustule. Thus, "pustule" specifically captures the swelling and nature of pus accumulation, making it the correct choice in this context.