**Core Concept**
Chronic rheumatic fever is a complication of acute rheumatic fever, which is an autoimmune inflammatory process triggered by group A beta-hemolytic streptococcal infection. The disease primarily affects the heart valves, leading to valvulitis and subsequent scarring. The involvement of specific heart valves in chronic rheumatic fever is determined by their embryological origin, with the mitral valve being the most commonly affected.

**Why the Correct Answer is Right**
The mitral valve is the least involved valve in chronic rheumatic fever because it is a left-sided valve that develops from the left atrioventricular canal. In contrast, the other three valves (aortic, tricuspid, and pulmonary) are right-sided and develop from the right atrioventricular canal. The left-sided valves are more commonly affected in chronic rheumatic fever due to their embryological origin and the resulting differences in valve structure and function. The mitral valve's involvement is relatively less common, especially in comparison to the aortic valve.

**Why Each Wrong Option is Incorrect**
**Option A:** The aortic valve is actually one of the most commonly affected valves in chronic rheumatic fever, particularly the right coronary cusp. This is due to its location and the resulting differences in blood flow and pressure.

**Option B:** The tricuspid valve is also commonly affected in chronic rheumatic fever, although to a lesser extent than the mitral and aortic valves. This is because the tricuspid valve is a right-sided valve that is more susceptible to the inflammatory process.

**Option C:** The pulmonary valve is relatively spared in chronic rheumatic fever due to its lower pressure and flow, but it can still be affected in some cases. However, it is not the least involved valve in comparison to the mitral valve.

**Clinical Pearl / High-Yield Fact**
The Aschoff bodies, which are characteristic of rheumatic fever, are collections of immune cells and fibrin that form in the valve tissue. These bodies are a hallmark of the disease and can be seen on histopathological examination of affected valves.

**Correct Answer: D. Mitral valve**