**Core Concept**
Pericarditis with pericardial effusion can cause referred pain due to irritation of the phrenic nerve, which innervates the diaphragm and pericardium. This referred pain can radiate to the shoulder and neck region due to shared nerve roots.

**Why the Correct Answer is Right**
The phrenic nerve, responsible for innervating the diaphragm and pericardium, carries pain fibers from the pericardium. When the pericardium is inflamed, as in pericarditis, these pain fibers can be stimulated, leading to referred pain in the shoulder and neck region. This phenomenon is due to the convergence of visceral and somatic pain fibers at the spinal cord level, specifically at the C3-C5 nerve roots.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might seem plausible, but the shoulder pain in pericarditis is not primarily due to referred pain from the myocardium. The myocardium is innervated by the ventricular nerves, which are not directly involved in the referred pain phenomenon.
**Option B:** This option is incorrect because the pain from pericarditis is not typically due to an inflammatory response in the subscapular region. While the subscapular region can be a source of referred pain, it is not the primary mechanism in pericarditis.
**Option C:** This option is incorrect because the pain from pericarditis is not primarily due to an inflammatory response in the rotator cuff. The rotator cuff can be a source of referred pain, but it is not the primary mechanism in pericarditis.

**Clinical Pearl / High-Yield Fact**
Referred pain from pericarditis can be remembered using the mnemonic "C3-C5," which refers to the spinal nerve roots involved in the convergence of visceral and somatic pain fibers. This is a classic exam trap, and students should be aware of the shared nerve roots involved in referred pain phenomena.

**Correct Answer: C. Phrenic nerve.**