**Core Concept**
Deep vein thrombosis (DVT) is a significant risk in patients undergoing major orthopedic surgery, particularly after total knee replacement. This is due to prolonged immobilization, surgical trauma, and hypercoagulability associated with the postoperative state.

**Why the Correct Answer is Right**
The patient's symptoms of calf pain and swelling are suggestive of a DVT. The subsequent development of breathlessness and sudden death is likely due to a pulmonary embolism (PE), which is a common complication of DVT. This is because the thrombus breaks loose and travels through the venous system to the lungs, causing a blockage in the pulmonary arteries. This can lead to acute right heart failure, hypoxia, and eventually death. The pathophysiology involves the activation of coagulation cascade, platelet aggregation, and the formation of a blood clot in the deep veins of the leg.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option is likely to be incorrect as it doesn't address the probable cause of the patient's symptoms. While cardiac arrhythmias can cause sudden death, they don't account for the preceding symptoms of calf pain and swelling.
* **Option B:** This option is incorrect as it doesn't consider the postoperative context. While sepsis is a possible cause of sudden death, it would typically present with fever, chills, and other signs of infection.
* **Option C:** This option is incorrect as it doesn't take into account the patient's recent surgery. While cardiac arrest can cause sudden death, it would typically be preceded by signs of cardiac dysfunction.

**Clinical Pearl / High-Yield Fact**
The Wells' score is a clinical prediction rule for DVT, which includes risk factors such as recent surgery, immobilization, and a history of previous DVT or PE. A score of 1.5 or higher is considered positive for DVT.

**Correct Answer:** D.