**Core Concept**
Acute rheumatic carditis is a complication of acute rheumatic fever, characterized by inflammation of the heart tissue. The histological findings in acute rheumatic carditis are crucial for diagnosis and understanding the pathophysiology of this condition.

**Why the Correct Answer is Right**
The most characteristic histological finding of acute rheumatic carditis is the presence of Aschoff bodies, which are granulomatous lesions consisting of an inner zone of fibrinoid necrosis surrounded by an outer zone of inflammatory cells, including lymphocytes, plasma cells, and macrophages. The formation of Aschoff bodies is thought to be mediated by an autoimmune response to Group A beta-hemolytic streptococcal infections. The Aschoff bodies are a hallmark of rheumatic carditis and are often found in the myocardium, endocardium, and pericardium.

**Why Each Wrong Option is Incorrect**
**Option A:** Fibrosis is a common finding in chronic rheumatic heart disease, but it is not the most characteristic histological finding of acute rheumatic carditis.
**Option B:** Infiltration of inflammatory cells is a feature of rheumatic carditis, but it is not specific to acute rheumatic carditis and can be seen in other conditions.
**Option C:** Fibrinoid necrosis is a feature of Aschoff bodies, but it is not the most characteristic histological finding of acute rheumatic carditis.

**Clinical Pearl / High-Yield Fact**
Aschoff bodies are a key diagnostic feature of acute rheumatic carditis and can be seen in the myocardium, endocardium, and pericardium. Remember the acronym "FIRE" to recall the components of Aschoff bodies: Fibrinoid necrosis, Inflammatory cells, Rheumatic carditis, and Endocardial involvement.

**Correct Answer: C. Fibrinoid necrosis and inflammatory cells.**