Which of the following is NOT a primary electrolyte needed for fluid balance?

Calcium is not considered a primary electrolyte in the context of fluid balance. The primary electrolytes crucial for maintaining fluid balance in the body are sodium, potassium, and chloride.

Sodium plays a vital role in regulating extracellular fluid volume and maintains osmotic pressure; it is pivotal for proper nerve function and muscle contraction. Potassium is essential for maintaining intracellular fluid balance and is critical for normal cellular function, particularly in nerve and muscle cells. Chloride, often associated with sodium, helps to maintain osmotic pressure and is involved in the production of gastric acid.

While calcium is an important mineral for many bodily functions, including bone health, muscle contraction, and signaling pathways, its role is not directly tied to fluid balance in the way the other electrolytes are. Thus, calcium is categorized separately, making it the correct answer in identifying which is not a primary electrolyte for fluid balance.