What disease process is characterized by a slow-growing abnormal sac of fluid within body tissues?

A cyst is indeed characterized by a slow-growing abnormal sac filled with liquid or semi-solid material that forms within body tissues. Cysts can occur in various parts of the body and may vary in size. They are generally enclosed in a membrane and can be found in organs, tissues, or even on the skin. Unlike benign tumors—which are solid masses—cysts are specifically fluid-filled, making this option the correct one.

While benign tumors also grow slowly, they do not typically have the fluid-filled sac criterion. Infections involve pathogenic organisms that provoke inflammation and may lead to fluid accumulation but are not defined as sacs themselves. Ulcers refer to open sores or lesions on the skin or mucous membranes, which do not correlate with the description of a fluid-filled sac. Therefore, the defining characteristics of a cyst align perfectly with the question provided, confirming it as the correct answer.