**Question:** In Rheumatic fever, fibrinoid necrosis occurs in

A. Endocardium
B. Arteries
C. Myocardium
D. Vessel walls

**Correct Answer:** A. Endocardium

**Core Concept:** Rheumatic fever is a post-infectious complication of group A Streptococcal pharyngitis (strep throat). It results from an abnormal immune response to the streptococcal antigen, leading to damage to heart, joints, skin, and other organs. Fibrinoid necrosis is a pathological process characterized by the destruction of the extracellular matrix, mainly composed of fibrin and other proteins, leading to tissue damage.

**Why the Correct Answer is Right:** In Rheumatic fever, the heart is most commonly affected, leading to rheumatic heart disease (RHD). The endocardium is the innermost layer of the heart valves, specifically the aortic and mitral valves. The presence of fibrinoid necrosis in endocardium is a hallmark of RHD, indicating inflammation, immune response, and subsequent tissue damage.

**Why Each Wrong Option is Incorrect:**

A. Fibrinoid necrosis can also occur in arteries (option B) and vessel walls (option D), but these options are not specific to Rheumatic fever.

B. While myocardial involvement (option C) can occur in severe cases of rheumatic fever, the primary focus of the question is fibrinoid necrosis, which is more prevalent in the endocardium.

**Clinical Pearl:** Understanding the specific pathological processes in Rheumatic fever can help with differential diagnosis and treatment planning. Fibrinoid necrosis in the endocardium is a critical clue to diagnose RHD, while other options represent non-specific findings in various cardiovascular diseases.