Intracellular fluid is best described as which?

• A. Fluid outside cells, making up about one-third of total body fluid and rich in sodium.
• B. Fluid in lymphatic vessels.
• C. Fluid in intercellular spaces between tissues.
• D. Fluid within cells, making up about two-thirds of total body fluid and rich in potassium.

Answer: (D)

The main idea being tested is where most of the body's water is located and its typical ionic composition. Intracellular fluid is the fluid inside the cells, and it is the larger of the two main compartments of body water—about two-thirds of total body fluid. Its composition is rich in potassium and phosphate and relatively low in sodium, which helps support cellular processes, enzyme function, and membrane potentials. This contrasts with extracellular fluid, which surrounds cells and is sodium-rich; components like plasma, interstitial fluid, and lymph are all part of extracellular fluid. Hence, the description that fits best is fluid within cells, making up about two-thirds of total body fluid and rich in potassium.