**Core Concept**
Pulmonary embolism is a blockage of an artery in the lungs caused by a dislodged blood clot, typically originating from the deep veins of the legs. This condition is a type of thromboembolic event that requires prompt diagnosis and treatment to prevent further complications.

**Why the Correct Answer is Right**
The most common source of pulmonary embolism is the deep vein thrombosis (DVT) in the lower extremities, particularly in the femoral and popliteal veins. The blood clots in these veins can break loose and travel through the bloodstream to the lungs, causing an embolism. This process is facilitated by the formation of a thrombus in the deep veins, which is often triggered by factors such as immobility, surgery, or malignancy.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a relevant option, as the question is asking for the most common source of pulmonary embolism.
**Option B:** While pelvic veins can be a source of pulmonary embolism, they are not the most common source.
**Option C:** The superior vena cava can be a source of pulmonary embolism, but it is not the most common source.

**Clinical Pearl / High-Yield Fact**
The most effective strategy for preventing pulmonary embolism is the use of mechanical prophylaxis, such as intermittent pneumatic compression devices or graduated compression stockings, in combination with anticoagulant therapy, particularly in high-risk patients.

**Correct Answer:** D. Deep vein thrombosis (DVT) in the lower extremities, particularly in the femoral and popliteal veins.