**Core Concept**
Pulmonary embolism (PE) is a blockage of an artery in the lungs, typically caused by a dislodged blood clot originating from a distant site. The underlying principle being tested here is the common source of these emboli.

**Why the Correct Answer is Right**
The most common cause of pulmonary embolism is deep vein thrombosis (DVT) in the legs. DVT occurs when a blood clot forms in the deep veins of the legs due to stasis, hypercoagulability, or vascular damage. These blood clots can break loose and travel to the lungs, causing a pulmonary embolism. The pathophysiology involves the activation of the coagulation cascade, platelet aggregation, and the formation of a fibrin clot.

**Why Each Wrong Option is Incorrect**
**Option A:** While cancer can increase the risk of DVT and PE, it is not the most common cause of PE.
**Option B:** Cardiac arrhythmias can contribute to the formation of blood clots, but they are not the primary cause of PE.
**Option C:** Pulmonary hypertension is a risk factor for PE, but it is not the most common cause.

**Clinical Pearl / High-Yield Fact**
The Wells' criteria, a clinical prediction rule for DVT, includes signs such as recent surgery, immobilization, or cancer as risk factors.

**Correct Answer:** D. DVT in the legs.