The commonest cause of pulmonary embolism is
**Core Concept**
Deep vein thrombosis (DVT) is the underlying condition that leads to pulmonary embolism (PE). The formation of a blood clot in the deep veins of the legs, typically in the femoral or popliteal veins, can break loose and travel to the lungs, causing a blockage in the pulmonary arteries.
**Why the Correct Answer is Right**
The most common cause of DVT is immobility, which leads to stasis of blood flow. This stasis, combined with the coagulability of blood, increases the risk of clot formation. The most common location for DVT is the lower extremities, particularly the femoral and popliteal veins. The clots formed in these veins can dislodge and travel to the lungs, resulting in a PE.
**Why Each Wrong Option is Incorrect**
* **Option A:** While smoking is a significant risk factor for many cardiovascular diseases, it is not the most common cause of DVT or PE.
* **Option B:** Obesity is a risk factor for DVT and PE, but it is not the most common underlying cause.
* **Option D:** While cancer can increase the risk of DVT and PE, it is not the most common cause of these conditions.
**Clinical Pearl / High-Yield Fact**
The Wells' criteria are a widely used clinical prediction rule for diagnosing DVT. The criteria include clinical signs and symptoms such as leg swelling, redness, and warmth, as well as risk factors like immobility and cancer.
**Correct Answer: C. DVT is the correct answer.