**Core Concept**
Hypercapnic respiratory failure is a condition characterized by the inability of the lungs to effectively remove carbon dioxide (CO2) from the blood, leading to elevated levels of CO2 in the blood (hypercapnia). This can result from various pulmonary and neuromuscular disorders.

**Why the Correct Answer is Right**
The most common cause of hypercapnic respiratory failure is chronic obstructive pulmonary disease (COPD). COPD is a progressive lung disease that makes it difficult to breathe, leading to impaired gas exchange. In COPD, the airways are narrowed and inflamed, resulting in air trapping and a decrease in lung volumes. This impairs the lungs' ability to remove CO2 from the blood, leading to hypercapnia. The primary pathophysiological mechanism in COPD is the destruction of alveolar walls and the formation of bullae, which reduces the surface area available for gas exchange.

**Why Each Wrong Option is Incorrect**
**Option A:** Congestive heart failure is a common cause of hypoxemic respiratory failure, not hypercapnic respiratory failure. While it can lead to pulmonary edema, which may impair gas exchange, it is not the primary cause of hypercapnic respiratory failure.
**Option B:** Pneumonia is a common cause of hypoxemic respiratory failure, and while it can lead to hypercapnia in severe cases, it is not the most common cause of hypercapnic respiratory failure.
**Option C:** Asthma is a common cause of hypoxemic respiratory failure, particularly in acute exacerbations, but it is not the most common cause of hypercapnic respiratory failure.

**Clinical Pearl / High-Yield Fact**
In patients with COPD, the use of non-invasive ventilation (NIV) can help improve respiratory muscle function and reduce the work of breathing, thereby improving gas exchange and reducing the risk of hypercapnic respiratory failure.

**Correct Answer:** C.