What is ascites?

Ascites refers specifically to the accumulation of fluid in the abdominal cavity. This condition is often associated with liver disease, heart failure, infection, or malignancies. The pathophysiological mechanisms include increased pressure in the blood vessels of the liver or low levels of proteins that help keep fluid in the bloodstream, resulting in fluid leaking into the surrounding abdominal area.

The other options describe different pathological conditions. Fluid accumulation in the lungs pertains to pulmonary edema, while fluid under the skin aligns with edema or localized swelling, and fluid accumulation in muscle tissue would relate to compartment syndrome or other muscular conditions. Thus, the definition of ascites is distinctly tied to the abdomen, making the correct answer clear.