**Core Concept**
Rheumatic carditis refers to the inflammation of the heart muscle, valves, and surrounding tissues, which is a complication of rheumatic fever. This condition involves the simultaneous involvement of multiple cardiac structures, leading to a complex pathological process.

**Why the Correct Answer is Right**
The characteristic feature of rheumatic carditis is the involvement of all three layers of the heart: the endocardium (innermost layer), the myocardium (middle layer), and the pericardium (outermost layer). This simultaneous involvement of the endocardium, myocardium, and pericardium is known as pancarditis. The inflammation of these layers can lead to various cardiac complications, including valve damage, heart failure, and arrhythmias. The autoimmune response triggered by the Group A beta-hemolytic streptococcus infection is thought to be the underlying mechanism of rheumatic carditis.

**Why Each Wrong Option is Incorrect**
**Option A:** Pericarditis, which is the inflammation of the pericardium, is a possible manifestation of rheumatic carditis but not its characteristic feature. Pericarditis can occur in isolation or as part of the pancarditis.
**Option B:** Endocarditis, which is the inflammation of the endocardium, is also a possible manifestation of rheumatic carditis but not its characteristic feature. Endocarditis can occur in isolation or as part of the pancarditis.
**Option C:** Myocarditis, which is the inflammation of the myocardium, is a possible manifestation of rheumatic carditis but not its characteristic feature. Myocarditis can occur in isolation or as part of the pancarditis.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that rheumatic carditis is a multifaceted condition that affects all three layers of the heart. The presence of pancarditis is a critical diagnostic clue for rheumatic carditis, and its recognition can guide further management and treatment.

**✓ Correct Answer: D. Pancarditis**