**Core Concept**
Deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, typically in the legs. It occurs when there's a combination of factors that lead to blood stasis, endothelial injury, and hypercoagulability. The formation of DVT is often a result of an imbalance between pro-coagulant and anti-coagulant forces.

**Why the Correct Answer is Right**
The correct answer is related to the most common cause of DVT, which is **immobility**. When a person is immobile, blood flow in the deep veins is reduced, leading to stasis and a higher risk of clot formation. This is often seen in patients who are bedridden, have undergone major surgery, or have a history of trauma. The **Virchow's triad** also plays a crucial role in the pathogenesis of DVT, which includes blood stasis, endothelial injury, and hypercoagulability. Immobility increases the risk of blood stasis, making it a significant risk factor for DVT.

**Why Each Wrong Option is Incorrect**
* **Option A:** While **cancer** can increase the risk of DVT, it is not the most common cause.
* **Option B:** **Pregnancy** is a risk factor for DVT, but it is not the most common cause.
* **Option C:** **Infection** can increase the risk of DVT, but it is not the most common cause.

**Clinical Pearl / High-Yield Fact**
The **Well's score** is a useful tool to predict the risk of DVT in hospitalized patients. It takes into account the patient's history of DVT, cancer, and immobility, as well as other risk factors. A score of 1 or more indicates a high risk of DVT.

**Correct Answer: A. Immobility**