**Core Concept**
The question is testing the student's understanding of the most common source of embolism, which is typically related to the venous system. In medical terminology, embolism refers to the blockage of a blood vessel by an embolus, which can be a blood clot, air bubble, or other material.

**Why the Correct Answer is Right**
The correct answer is related to the fact that venous thromboembolism (VTE) is a leading cause of morbidity and mortality worldwide. VTE occurs when a blood clot forms in a vein, breaks loose, and travels through the bloodstream, eventually lodging in a pulmonary artery, which can cause a pulmonary embolism. The most common source of these clots is the deep veins of the legs, particularly the deep vein thrombosis (DVT) of the lower extremities.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a common source of embolism. While it is true that air bubbles can cause embolism, they are not the most common source.
**Option B:** This option is incorrect because it is not directly related to embolism. While cardiac problems can lead to embolism, they are not the most common source.
**Option C:** This option is incorrect because it is not a specific source of embolism. While the heart can be a source of embolism, it is not the most common source.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the most common source of embolism is the venous system, particularly the deep veins of the legs. This is why patients with a history of DVT or VTE are at increased risk of developing pulmonary embolism.

**Correct Answer:** C.