**Core Concept**
Rheumatic fever is an autoimmune inflammatory process that primarily affects the heart valves, leading to their scarring and dysfunction. The involvement of heart valves in rheumatic fever is largely determined by the distribution of the inflammatory process and the mechanical stresses on the valves.

**Why the Correct Answer is Right**
The pulmonary valve is the least commonly affected valve in rheumatic fever due to its unique anatomical position and the direction of blood flow. Unlike the other valves, which are subject to high-pressure and turbulent blood flow, the pulmonary valve experiences relatively low-pressure and laminar flow, making it less susceptible to rheumatic inflammation. The inflammation in rheumatic fever typically targets the valves that are exposed to higher pressures, such as the mitral and aortic valves.

**Why Each Wrong Option is Incorrect**
**Option A:** The aortic valve is one of the most commonly affected valves in rheumatic fever, particularly in its acute phase. The inflammation often involves the valve leaflets and the surrounding annulus, leading to scarring and stenosis.

**Option B:** The tricuspid valve is occasionally affected in rheumatic fever, but it is less common than the involvement of the mitral and aortic valves. However, it is still more frequently affected than the pulmonary valve.

**Option C:** The mitral valve is the most commonly affected valve in rheumatic fever, particularly in its chronic phase. The inflammation often leads to valve leaflet thickening, shortening, and scarring, resulting in stenosis or regurgitation.

**Clinical Pearl / High-Yield Fact**
The Jones criteria are a set of guidelines used to diagnose rheumatic fever, which include major and minor criteria. The major criteria include carditis, arthritis, chorea, erythema marginatum, and subcutaneous nodules, while the minor criteria include fever, arthralgia, elevated erythrocyte sedimentation rate (ESR), and elevated C-reactive protein (CRP).

**Correct Answer:**
✓ Correct Answer: D. Pulmonary valve