**Core Concept:** Rheumatic fever (RF) is an immune-mediated complication of streptococcal infection, primarily affecting the heart, joints, skin, and central nervous system. The MC valve involved is the mitral valve. RF can lead to valvular damage and dysfunction.

**Why the Correct Answer is Right:**
Rheumatic fever is a post-infectious disease caused by Group A beta-hemolytic Streptococci (GABHS) infection, which triggers an autoimmune response. Among the heart involvement in RF, mitral valve is most commonly affected due to its larger size and closer proximity to the heart's left atrium. The immune system mistakenly attacks the heart tissue, leading to inflammation and damage to the mitral valve (mitral valve regurgitation or mitral valve stenosis).

**Why Each Wrong Option is Incorrect:**
A. Tricuspid valve: Although RF can affect any heart valve, the correct answer is the mitral valve as it is more commonly involved.
B. Aortic valve: Similar to option A, the aortic valve is not the MC valve involved in RF.
C. Pulmonary valve: Similar to options A and B, the pulmonary valve is not the MC valve affected in RF.
D. All valves: While RF can affect multiple heart valves, the MC valve involved is the mitral valve, as mentioned earlier.

**Clinical Pearl:**
Rheumatic fever is a significant global health issue, particularly in low-income countries. Early diagnosis and treatment of streptococcal infections, particularly group A streptococcal pharyngitis (strep throat), can prevent the development of RF and its sequelae like rheumatic heart disease (RHD).

**Correct Answer:** C. Mitral valve