**Core Concept**
The diagnostic feature of Rheumatic Heart Disease (RHD) is closely related to the pathophysiological changes that occur in the heart as a result of acute rheumatic fever (ARF). RHD is characterized by inflammation of the heart valves, particularly the mitral valve, leading to scarring, thickening, and eventual deformation of the valve leaflets.

**Why the Correct Answer is Right**
The correct diagnostic feature of RHD is the presence of _Aschoff bodies_, which are characteristic pathological lesions found in the heart tissue of individuals with ARF. Aschoff bodies are granulomatous lesions composed of an accumulation of immune cells, including macrophages, lymphocytes, and plasma cells, which infiltrate the heart muscle and valves. This inflammatory response leads to the production of fibrosis and scarring, ultimately resulting in the deformation of heart valves. The presence of Aschoff bodies is a key diagnostic feature of RHD and is often detected through histopathological examination of heart tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** While valve thickening and scarring are associated with RHD, they are not the specific diagnostic feature being referred to in this question.
**Option B:** Echocardiography is a diagnostic tool used to detect RHD, but it is not a specific pathological feature of the disease.
**Option C:** Cardiac arrhythmias can occur in RHD due to the deformation of heart valves, but they are not a diagnostic feature of the disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Aschoff bodies are a hallmark of ARF and RHD, and their presence can be confirmed through histopathological examination of heart tissue.

**Correct Answer: C. Aschoff bodies.