**Core Concept**
Deep vein thrombosis (DVT) is a condition where a blood clot forms in the deep veins of the body, often in the legs. This condition can lead to pulmonary embolism (PE), a blockage of an artery in the lungs, which can be life-threatening if not promptly treated.

**Why the Correct Answer is Right**
The patient's symptoms of calf pain and swelling in the leg, followed by sudden breathlessness, are classic signs of a pulmonary embolism. The most likely cause of this condition is a deep vein thrombosis that has broken loose and traveled to the lungs. This is known as a pulmonary embolism, which can cause sudden death if not treated promptly. The pathophysiology behind this involves the formation of a blood clot in the deep veins, which can dislodge and travel through the bloodstream to the lungs, causing a blockage.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the symptoms described and is likely unrelated to the patient's condition.
* **Option B:** While a pulmonary embolism can be a complication of surgery, it is not the most likely cause of sudden death in this patient, given the symptoms of calf pain and swelling.
* **Option D:** This option is not a plausible cause of sudden death in this patient, given the symptoms described.

**Clinical Pearl / High-Yield Fact**
The Wells' score is a clinical prediction rule used to estimate the probability of pulmonary embolism in patients with symptoms of DVT or PE. It takes into account factors such as clinical signs and symptoms, D-dimer levels, and risk factors for DVT or PE.

**Correct Answer: C. Pulmonary Embolism**