**Core Concept**
Metastatic calcification is a form of calcium deposition in previously normal tissues, resulting from elevated serum calcium levels. This condition can occur due to various factors, including hypercalcemia of malignancy, primary hyperparathyroidism, and vitamin D intoxication. The process involves the precipitation of calcium salts, such as calcium phosphate or calcium carbonate, in tissues like the lungs, kidneys, stomach, and other organs.

**Why the Correct Answer is Right**
Cardiac valves are not typically affected by metastatic calcification. The primary sites for metastatic calcification are the lungs, kidneys, and stomach, where the deposition of calcium salts can lead to tissue damage and dysfunction. The lungs are particularly susceptible due to the high concentration of surfactant molecules, which can facilitate calcium salt precipitation. In contrast, the cardiac valves are more prone to dystrophic calcification, a different process involving abnormal calcium deposition in damaged or degenerating tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** The lungs are a common site for metastatic calcification, particularly in the setting of hypercalcemia of malignancy or primary hyperparathyroidism.
**Option B:** The kidneys are also frequently affected by metastatic calcification, which can lead to nephrocalcinosis and renal dysfunction.
**Option C:** The gastric mucosa can be a site for metastatic calcification, particularly in the context of gastric carcinoma or other conditions associated with hypercalcemia.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with hypercalcemia, it's essential to consider the presence of metastatic calcification as a potential cause of tissue damage and organ dysfunction. The lungs and kidneys are particularly vulnerable to this process, and identifying these affected organs can help guide treatment decisions.

✓ Correct Answer: D. Cardiac valves