**Core Concept**
Pulmonary embolism (PE) is a blockage of an artery in the lungs by a substance that has traveled from elsewhere in the body, typically a blood clot. The risk of PE is increased by conditions that promote blood clot formation, slow blood flow, or damage the blood vessels.

**Why the Correct Answer is Right**
Several conditions can increase the risk of pulmonary embolism by promoting blood clot formation or damaging blood vessels. These include:
- **Option C:** Cancer, particularly pancreatic cancer, can increase the risk of PE by promoting blood clot formation due to the release of pro-coagulant substances.
- **Option D:** Obesity can increase the risk of PE by promoting blood clot formation due to increased blood stasis.
- **Option A:** Long-distance travel, especially with prolonged immobility, can increase the risk of PE by promoting blood clot formation due to blood stasis.
- **Option B:** Atrial fibrillation, a type of irregular heartbeat, can increase the risk of PE by allowing blood to pool in the heart and promoting blood clot formation.

**Why Each Wrong Option is Incorrect**
- **Option A:** While long-distance travel can increase the risk of PE, it is not the most significant risk factor among those listed. However, it is still a relevant consideration.
- **Option B:** Atrial fibrillation is a well-established risk factor for PE, but it is not the best choice as the exception in this list.
- **Option C:** Cancer, particularly pancreatic cancer, is a significant risk factor for PE, making it an incorrect choice as the exception.

**Clinical Pearl / High-Yield Fact**
Remember the "Virchow's triad" when considering the risk of pulmonary embolism: blood stasis, hypercoagulability, and vascular injury. Conditions that promote any of these factors can increase the risk of PE.

**Correct Answer: A.**