**Core Concept:** The question is asking about ions, which are charged particles that play a crucial role in maintaining cellular function and homeostasis. Electrolytes are inorganic ions, dissolved in body fluids, that are essential for various physiological processes, including nerve and muscle function, osmoregulation, and acid-base balance. In this context, we are considering the diffusibility of ions to understand their relative importance in various physiological processes and their ability to cross cellular membranes.

**Why the Correct Answer is Right:** The correct answer, **C. Calcium (Ca²⁺)**, is right because calcium is highly bound to proteins and is not easily soluble in water. This makes it less likely to diffuse across cell membranes compared to other ions.

**Why Each Wrong Option is Incorrect:**

A. **Magnesium (Mg²⁺)** is incorrect because magnesium is less tightly bound to proteins than calcium, so it has a higher chance of being diffusible across cell membranes.

B. **Potassium (K⁺)** is incorrect because potassium is also tightly bound to proteins, but less so than calcium. Its higher diffusibility compared to calcium makes it incorrect in this context.

D. **Sodium (Na¹⁺)** is incorrect because sodium is highly soluble in water and binds loosely to proteins. This makes it more easily diffusible across cell membranes compared to calcium.

**Clinical Pearl:** Understanding the diffusibility of ions is crucial in understanding various physiological processes and pathophysiology. For example, a decreased calcium concentration in plasma can lead to hypocalcemia, which may cause tetany, muscle cramps, and seizures. Conversely, high calcium levels can cause hypercalcemia, leading to kidney stones, osteoporosis, and kidney damage.

In summary, understanding the relationship between ion binding to proteins and their diffusibility is essential for making accurate diagnoses and formulating effective treatment plans in clinical practice.