What is the most prevalent cation in intracellular fluid?

• A. Potassium
• B. Sodium
• C. Calcium
• D. Magnesium

Answer: (A)

Potassium is the most prevalent intracellular cation because cells actively maintain a high concentration of K+ inside while keeping other cations lower in number inside the cytosol. This balance is achieved mainly through the Na+/K+-ATPase pump, which moves three Na+ ions out of the cell for every two K+ ions it brings in, using ATP. This pump sets up and sustains the steep gradient: high intracellular K+ and low intracellular Na+. Potassium then partly leaks out through K+ channels, but the pump continuously restores it, keeping the inside of the cell negatively charged relative to the outside and supporting the resting membrane potential that is crucial for excitability.

Sodium, in contrast, is the major extracellular cation, not intracellular. Calcium inside the cell is kept very low because it's a potent signaling ion, tightly buffered and sequestered. Magnesium is present intracellularly as well, but it is not the dominant intracellular cation. So, the most prevalent intracellular cation is potassium.