**Core Concept**
Pulmonary embolism (PE) occurs when a blood clot (thrombus) breaks loose from its site of formation and travels through the bloodstream to lodge in one of the pulmonary arteries, blocking blood flow to the lung tissue. The commonest cause of pulmonary embolism is related to venous thromboembolism (VTE), which is a multifactorial process involving both local and systemic factors.

**Why the Correct Answer is Right**
The correct answer involves a common underlying condition that predisposes individuals to VTE. The formation of a blood clot in the deep veins of the legs (deep vein thrombosis, DVT) is a common precursor to pulmonary embolism. This occurs due to a combination of factors including stasis (reduced blood flow), hypercoagulability (increased clotting tendency), and endothelial injury (damage to the inner lining of blood vessels). The most common underlying condition that contributes to these factors is **malignancy**. Tumors can release pro-coagulant substances that promote clot formation, and the resulting DVT can break loose and travel to the lungs, causing a pulmonary embolism.

**Why Each Wrong Option is Incorrect**
* **Option A:** While immobilization is a risk factor for VTE, it is not the most common cause of pulmonary embolism. Immobilization can lead to stasis, which contributes to clot formation, but it is not a direct cause.
* **Option B:** Inherited thrombophilias, such as factor V Leiden, can increase the risk of VTE. However, they are not the most common cause of pulmonary embolism.
* **Option C:** Antithrombin deficiency is a rare condition that increases the risk of VTE. However, it is not the most common cause of pulmonary embolism.

**Clinical Pearl / High-Yield Fact**
The most common site of DVT formation is the **popliteal vein**, which is located behind the knee. This is because the popliteal vein is a common site of stasis and endothelial injury due to the anatomy of the knee joint.

**Correct Answer:** C. Malignancy