**Core Concept**
Pericarditis is an inflammation of the pericardium, the fibrous sac surrounding the heart, often causing chest pain due to irritation of the visceral pericardium. The pain from pericarditis is typically referred through the phrenic nerve, which innervates the pericardium.

**Why the Correct Answer is Right**
The phrenic nerve arises from the cervical spine (C3-C5 roots) and descends through the thoracic cavity to innervate the pericardium, diaphragm, and mediastinal pleura. The visceral pericardium is innervated by the phrenic nerve, which mediates pain sensation from the pericardium. When the pericardium is inflamed, the phrenic nerve transmits pain signals to the central nervous system, resulting in referred pain.

**Why Each Wrong Option is Incorrect**
**Option A:** The intercostal nerves are responsible for innervating the intercostal spaces and do not mediate pain from the pericardium.
**Option B:** The vagus nerve has various parasympathetic functions, including regulation of heart rate, but is not involved in pain sensation from the pericardium.
**Option C:** The sympathetic nervous system is involved in the "fight or flight" response, but does not mediate pain sensation from the pericardium.

**Clinical Pearl / High-Yield Fact**
The pain from pericarditis is often exacerbated by deep breathing or coughing, as these actions increase the pressure within the thoracic cavity, irritating the inflamed pericardium.

**Correct Answer:** D. The phrenic nerve.