Which is a secondary intracellular cation?

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

Answer: (A)

The key idea is how cations are distributed inside and outside the cell. Potassium is the dominant intracellular cation, helping set the resting membrane potential and drive many cellular processes. Magnesium comes next in abundance inside the cell, making it the secondary intracellular cation. It serves as a crucial cofactor for many enzymes and is essential for ATP stability and function. Sodium is primarily outside the cell, while calcium inside is kept very low under resting conditions and mainly acts as a signaling ion when released. Because of its substantial intracellular presence and broad roles in enzyme activity and ATP binding, magnesium is the best choice as the secondary intracellular cation.