**Core Concept**
The pericardium is a fibroserous sac surrounding the heart, composed of two layers: the fibrous pericardium and the serous pericardium. Pain sensitivity is a critical aspect of pericardial disease, as it helps diagnose conditions such as pericarditis.

**Why the Correct Answer is Right**
The correct answer is the fibrous pericardium. This is because the fibrous pericardium is a dense, avascular layer that lacks pain receptors. In contrast, the serous pericardium, which is the inner layer of the pericardium, contains nerves that transmit pain signals to the brain. This is important to note because conditions affecting the serous pericardium, such as pericarditis, can cause chest pain.

**Why Each Wrong Option is Incorrect**
**Option A:** The epicardium is a layer of the pericardium, but it is not the correct answer because it is part of the serous pericardium, which is sensitive to pain.

**Option B:** The parietal layer of the serous pericardium is indeed sensitive to pain, as it contains nerves that transmit pain signals to the brain.

**Option C:** The visceral layer of the serous pericardium is also sensitive to pain, as it contains nerves that transmit pain signals to the brain.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that conditions affecting the serous pericardium can cause chest pain, whereas conditions affecting the fibrous pericardium are often asymptomatic or may present with non-specific symptoms.

**Correct Answer: A. Fibrous pericardium.**