**Core Concept**
The phrenic nerve plays a crucial role in the transmission of pain from the pericardium to the central nervous system. The pericardium is a fibro-serous sac surrounding the heart, and its inflammation (pericarditis) can cause severe chest pain.

**Why the Correct Answer is Right**
The phrenic nerve is responsible for transmitting pain from the pericardium due to its close proximity to the heart and its innervation of the pericardial sac. The phrenic nerve arises from the cervical roots (C3-C5) and descends through the thoracic cavity, providing sensory innervation to the pericardium. When the pericardium is inflamed, the phrenic nerve transmits pain signals to the brain, resulting in referred pain in the chest.

**Why Each Wrong Option is Incorrect**
**Option B:** The superficial cardiac plexus is a collection of nerve fibers that provides sympathetic innervation to the heart, but it is not directly involved in the transmission of pain from the pericardium.

**Option C:** The deep cardiac plexus is a network of nerve fibers that provides parasympathetic innervation to the heart, but it is not responsible for pain transmission.

**Option D:** The vagus nerve provides parasympathetic innervation to various organs, including the heart, but it is not directly involved in the transmission of pain from the pericardium.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the phrenic nerve is responsible for transmitting pain from the pericardium, and its irritation can lead to referred pain in the chest. This knowledge is critical in diagnosing and managing patients with pericarditis and pericardial effusion.

**✓ Correct Answer: A. Phrenic nerve**