**Core Concept**
Rheumatic fever is an autoimmune inflammatory disorder that affects the heart, joints, skin, and brain. The disease is caused by a group A beta-hemolytic streptococcal infection, which triggers an immune response that mistakenly attacks the body's own tissues. In the heart, rheumatic fever causes inflammation of the valve leaflets, leading to scarring and potential valvular dysfunction.

**Why the Correct Answer is Right**
The pulmonary valve is not commonly involved in rheumatic fever because it is a relatively low-pressure valve compared to the left-sided heart valves (mitral and aortic). The immune response in rheumatic fever tends to target the mitral and aortic valves due to their higher pressure and stress. Additionally, the pulmonary valve is less exposed to the circulating immune complexes that contribute to the inflammatory process.

**Why Each Wrong Option is Incorrect**
**Option A:** The mitral valve is frequently involved in rheumatic fever due to its high pressure and exposure to circulating immune complexes.
**Option B:** The aortic valve is commonly affected by rheumatic fever, which can lead to aortic regurgitation or stenosis.
**Option D:** Although less common, the tricuspid valve can be involved in rheumatic fever, particularly in cases with right heart involvement.

**Clinical Pearl / High-Yield Fact**
Rheumatic fever often affects the mitral valve more frequently than the aortic valve, resulting in mitral stenosis being a common consequence.

**✓ Correct Answer: C. Pulmonary**