**Core Concept**
Metastatic calcification is a pathological process where calcium salts deposit in tissues due to an increase in serum calcium levels, often resulting from hyperparathyroidism, vitamin D toxicity, or other conditions that lead to hypercalcemia. This process is distinct from dystrophic calcification, which occurs in damaged or necrotic tissues.

**Why the Correct Answer is Right**
The renal tubules are particularly susceptible to metastatic calcification due to their high concentration of calcium ions and the presence of acid mucopolysaccharides, which can act as a nidus for calcium deposition. When serum calcium levels rise, calcium ions can diffuse into the renal tubular cells and form insoluble calcium salts, ultimately leading to calcification. This process can impair renal function and contribute to the development of renal failure. The renal tubules' high metabolic activity and the presence of calcium-rich fluids make them an ideal site for metastatic calcification.

**Why Each Wrong Option is Incorrect**
**Option A:** Corneal calcification is more commonly associated with dystrophic calcification, which occurs in damaged or degenerated tissues, rather than metastatic calcification.
**Option B:** Extensor tendons are not typically affected by metastatic calcification due to their low metabolic activity and lack of exposure to high levels of calcium ions.
**Option C:** Brain calcification is more commonly associated with conditions such as idiopathic calcification of basal ganglia, which is a separate entity from metastatic calcification.

**Clinical Pearl / High-Yield Fact**
Metastatic calcification can be distinguished from dystrophic calcification by the presence of hypercalcemia and the involvement of tissues with high metabolic activity, such as the renal tubules.

**✓ Correct Answer: D. Renal tubules**