**Core Concept**
Deep vein thrombosis (DVT) and pulmonary embolism (PE) are leading causes of preventable hospital deaths. After major surgeries, especially orthopedic procedures like total knee replacement, patients are at an increased risk of developing DVT due to prolonged immobilization and hypercoagulability.

**Why the Correct Answer is Right**
The patient's symptoms of calf pain and swelling are classic signs of DVT. The sudden onset of breathlessness indicates that the DVT has led to a pulmonary embolism, which is a life-threatening condition. The embolism obstructs blood flow to the lungs, causing hypoxia and eventually, death. The patient's sudden death in the ward is a direct consequence of the pulmonary embolism.

**Why Each Wrong Option is Incorrect**
**Option A:** Myocardial infarction can cause chest pain, breathlessness, and even sudden death, but it would not typically present with calf pain and swelling as initial symptoms.

**Option C:** Stroke can cause sudden neurological deficits, but it would not typically present with breathlessness or calf pain and swelling.

**Option D:** Acute Respiratory Distress Syndrome (ARDS) can cause breathlessness, but it would not typically present with calf pain and swelling as initial symptoms.

**Clinical Pearl / High-Yield Fact**
After major surgeries, especially orthopedic procedures, patients should be closely monitored for signs of DVT, such as calf pain and swelling, and should receive prophylactic anticoagulation therapy to prevent DVT and PE.

**✓ Correct Answer: B. Pulmonary embolism**