**Core Concept**
Pulmonary embolism (PE) is a life-threatening condition caused by a blockage in one of the pulmonary arteries, which can lead to impaired gas exchange and potentially fatal consequences. This blockage is typically a result of a dislodged blood clot that originates from the venous system.

**Why the Correct Answer is Right**
Deep vein thrombosis (DVT) of the leg is a common source of pulmonary embolism. The formation of a blood clot in the deep veins of the legs, often due to immobility, trauma, or hypercoagulability, can lead to the release of the clot into the venous system, eventually lodging in the pulmonary arteries. This is a well-documented phenomenon in medical literature, with the majority of pulmonary emboli originating from the lower extremities.

**Why Each Wrong Option is Incorrect**
**Option B:** Increase in pulmonary tension is not a direct cause of pulmonary embolism. While pulmonary hypertension can be a complication of chronic PE, it is not the underlying mechanism that leads to the formation of a pulmonary embolus.

**Option C:** Fracture of the pelvic region can increase the risk of developing DVT, which in turn can lead to PE. However, the fracture itself is not a direct cause of pulmonary embolism.

**Option D:** Heart disease is not a direct cause of pulmonary embolism. While certain heart conditions, such as atrial fibrillation, can increase the risk of clot formation, they are not the primary cause of PE.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that patients with a history of recent surgery, immobility, or trauma are at an increased risk of developing DVT and subsequently PE. A prompt diagnosis and treatment of DVT can prevent the development of pulmonary embolism.

**✓ Correct Answer: A. Deep vein thrombosis of leg**