Final answer:
A sinus/fistula refers to an anomalous connection between two epithelium-lined internal surfaces or between any internal surface and the body's exterior.
Step-by-step explanation:
A sinus/fistula is C. An abnormal connection between two epithelium-lined surfaces. A sinus can be a cavity or channel that allows the passage of fluid. In contrast, a fistula is an abnormal connection usually following disease or injury that creates a passage between two organs or between an organ and the surface of the body.
For instance, fistulas can occur in various parts of the body, such as between the intestines and the skin. Depending on their location, fistulas may allow the escape of fluids like pus which is a suppurative or pus-producing infection.
While discussing skin-related issues, other lesions include fluid- or pus-filled bumps on the skin, which are often the result of infection or inflammation. For example, an abscess is a pus-filled lesion, and a cyst is an encapsulated sac filled with fluid, semi-solid matter, or gas.
On the other hand, fluid-filled bumps on the skin can be a sign of conditions such as acne. Knowledge of skin lesions is important in health fields to diagnose and treat various conditions that manifest on the skin's surface.