**Core Concept**
Cor pulmonale is a type of heart disease characterized by enlargement and failure of the right ventricle, primarily caused by a primary disorder of the respiratory system. This condition is often the result of chronic high blood pressure in the pulmonary arteries, leading to right ventricular hypertrophy and eventual failure.

**Why the Correct Answer is Right**
Chronic obstructive pulmonary disease (COPD) is a common cause of cor pulmonale. In COPD, the lungs are unable to expand properly, leading to increased resistance in the pulmonary arteries. This increased resistance causes the right ventricle to work harder to pump blood through the lungs, resulting in right ventricular hypertrophy and eventual failure. The key mechanism involved is the increased pressure in the pulmonary arteries, which is sensed by the right ventricle, leading to compensatory hypertrophy and, ultimately, failure.

**Why Each Wrong Option is Incorrect**
**Option B:** Left ventricular failure is not a direct cause of cor pulmonale, although it can lead to secondary effects on the right ventricle. The correct answer is COPD affecting the right ventricle, not left ventricular failure.

**Option C:** Pericardial effusion can cause cardiac tamponade, which can lead to right ventricular failure, but it is not a primary cause of cor pulmonale.

**Option D:** While none of the above options are the primary cause of cor pulmonale, COPD affecting the right ventricle is a specific and correct answer.

**Clinical Pearl / High-Yield Fact**
The most common causes of cor pulmonale are COPD, pulmonary embolism, and pulmonary hypertension. These conditions can lead to right ventricular failure, which can have significant clinical implications, including dyspnea, fatigue, and syncope.

**✓ Correct Answer: A. COPD affecting right ventricle**