**Core Concept**
Acute cor pulmonale refers to a sudden onset right ventricular failure due to increased pressure and resistance in the pulmonary circulation. This condition often results from a primary pulmonary cause, leading to pulmonary hypertension and subsequent right ventricular dysfunction.

**Why the Correct Answer is Right**
The most common cause of acute cor pulmonale is a massive pulmonary embolism (PE), which is a blockage of an artery in the lungs by a blood clot. This sudden obstruction leads to a significant increase in pulmonary artery pressure, causing the right ventricle to fail and resulting in acute cor pulmonale. The mechanism involves the sudden increase in pulmonary vascular resistance, which overwhelms the right ventricle's ability to pump blood effectively.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chronic obstructive pulmonary disease (COPD) is a long-standing condition that may lead to chronic cor pulmonale, but it is not the most common cause of acute cor pulmonale.
* **Option B:** Left ventricular failure is a potential cause of cor pulmonale, but it is not typically considered acute cor pulmonale.
* **Option C:** Cardiac tamponade is a condition that can lead to right ventricular failure, but it is not the most common cause of acute cor pulmonale.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that massive pulmonary embolism is a medical emergency that requires prompt recognition and treatment to prevent mortality.

**Correct Answer:** C. Cardiac tamponade