## **Core Concept**
The clinical presentation of sudden massive swelling in one lower extremity during the last trimester of pregnancy strongly suggests **deep vein thrombosis (DVT)**, a condition characterized by the formation of a blood clot in a deep vein. Pregnancy is a hypercoagulable state due to increased levels of clotting factors and decreased levels of anticoagulant proteins, which predisposes women to venous thromboembolism.

## **Why the Correct Answer is Right**
The correct approach involves immediate diagnosis and treatment to prevent complications such as pulmonary embolism. The most appropriate initial step would include **compression ultrasound** (specifically, duplex ultrasound) as the diagnostic tool of choice for DVT in pregnancy due to its safety, accuracy, and lack of radiation. Anticoagulation therapy is typically initiated promptly if DVT is confirmed. **Option C** likely represents this approach.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option might suggest a less immediate or less specific approach, potentially leading to delays in diagnosis and treatment, which could be harmful given the risk of pulmonary embolism.
- **Option B:** While imaging and possibly a different initial management strategy could be considered, if it does not align with immediate diagnosis via ultrasound and initiation of anticoagulation, it would be less appropriate.
- **Option D:** This might imply a more invasive diagnostic approach or a treatment strategy that is not first-line for acute DVT in pregnancy.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **DVT in pregnancy should be highly suspected and promptly evaluated**, especially in the setting of risk factors such as recent surgery, trauma, immobility, or a history of thromboembolism. The **Wells score** can be helpful but might be less specific in pregnancy. Always consider the safety of both mother and fetus in diagnostic and therapeutic decisions.

## **Correct Answer: C.**