Okay, let's tackle this question. The user is asking about the commonest cause of pulmonary embolism. I remember that pulmonary embolism is usually due to a blood clot that travels to the lungs. The main source of these clots is the deep veins of the legs, right? So deep vein thrombosis (DVT) is the main cause. But wait, are there other sources? Like the pelvic veins or maybe the arms? But I think the deep veins of the lower extremities are the most common.

Now, the options aren't provided here, but the correct answer is supposed to be deep vein thrombosis. Let me check if there's any other common cause. I know that sometimes clots can come from other places, but DVT is the most frequent. The question is from CMC Vellore, so maybe they're testing knowledge of the typical source. Also, the clinical pearl here is that DVT and PE are parts of the same condition, venous thromboembolism. The mnemonic might be "Virchow's triad" for risk factors: stasis, hypercoagulability, endothelial injury. But the question is about the cause, not the risk factors. So the main answer is DVT. I should make sure that the explanation covers why DVT is the primary cause and why other options like fat embolism or air embolism are less common. Also, maybe mention that other options could be things like atherosclerosis or cancer, but those are risk factors, not the direct cause. So the key is that the embolus is from a clot in the deep veins, typically the legs.

**Core Concept**
Pulmonary embolism (PE) most commonly results from **deep vein thrombosis (DVT)** in the lower extremities. This is part of **venous thromboembolism (VTE)**, where a thrombus dislodges and lodges in pulmonary arteries, obstructing blood flow. The **Femoral, Popliteal, and Calf veins** are frequent sites of DVT formation.

**Why the Correct Answer is Right**
Deep vein thrombosis (DVT) accounts for **80-90% of PEs** due to anatomical proximity and venous return pathways. The thrombus in DVTs can fragment and travel via the inferior vena cava into the right heart, then to the pulmonary arteries. **Virchow’s triad** (stasis, hypercoagulability, endothelial injury) often underlies DVT formation, making it the primary source of emboli in clinical settings.

**Why Each Wrong Option is Incorrect**
**Option A:** Fat embolism (e.g., after fractures) is rare and typically causes smaller, less severe PEs.
**Option B:** Air embolism is uncommon and usually iatrogenic (e.g., during procedures).
**Option C:** Malignancy increases thrombosis risk but is not a direct cause of PE.
**Option D:** Atherosclerosis causes arterial emboli (e.g., in stroke), not venous thromboembolism.

**Clinical Pearl / High-Yield Fact**
Remember **"DVT → PE"** as a core clinical correlation. Always assess for DVT risk factors (immobility, surgery

✓ Correct Answer: C. Thrombus