Mineral functions — Which mineral is necessary for blood clotting, the activity of certain enzymes, and regulation of fluid movement across cell membranes?

Introduction / Context:
Calcium is best known for skeletal health, but it also plays vital roles in hemostasis, enzyme activation, and membrane physiology. Recognizing these non-skeletal functions is important for interpreting clinical signs of hypocalcemia and for understanding the broader roles of minerals in physiology.

Given Data / Assumptions:

• Calcium ions are required cofactors for several steps in the coagulation cascade.
• Ca2+ modulates many enzyme activities and cell signaling pathways.
• Cell membranes use Ca2+ gradients and binding to regulate permeability and transport processes.

Concept / Approach:
Associate each listed function with the mineral most directly responsible. The triad of coagulation, enzyme function, and membrane fluid regulation aligns squarely with calcium physiology, not with the other listed minerals.

Step-by-Step Solution:

Verification / Alternative check:
Laboratory anticoagulants chelate Ca2+ (e.g., citrate) to prevent clotting, demonstrating Ca2+ necessity.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming electrolytes like potassium or magnesium substitute for calcium in clotting; they do not.

Final Answer:
Calcium.