**Core Concept**
Calcification is the process of calcium deposition into a tissue or organ, leading to the formation of calcium salts. This process can occur in various contexts, including bone formation, stone formation, and pathological calcification in tissues such as the heart, kidneys, and lungs.

**Why the Correct Answer is Right**
Calcification is a complex process involving multiple biochemical pathways. One of the key regulators of calcification is the calcium-sensing receptor (CaSR), which is expressed on the surface of various cells, including osteoblasts, osteoclasts, and renal tubular cells. The CaSR plays a crucial role in maintaining calcium homeostasis by regulating calcium absorption, excretion, and bone resorption. In conditions of abnormal calcium levels, such as hypercalcemia, the CaSR can be activated or inhibited, leading to changes in calcification rates.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because calcification is not solely dependent on the presence of calcium ions. While calcium is a critical component of the calcification process, other factors such as phosphate, magnesium, and pH also play important roles.

**Option B:** Incorrect because calcification is not a passive process. While some forms of calcification, such as bone formation, may appear to be a passive process, others, such as pathological calcification, are actively regulated by cellular mechanisms.

**Option C:** Incorrect because calcification is not exclusive to bone. While calcification is a key component of bone formation, it can also occur in other tissues, including the kidneys, heart, and lungs.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that abnormal calcification can be a marker of underlying disease, such as hyperparathyroidism, vitamin D deficiency, or chronic kidney disease.

**Correct Answer: C. Calcification is not exclusive to bone.**