**Core Concept**
Rheumatic heart disease (RHD) is a complication of rheumatic fever, which can lead to valvular heart disease, particularly affecting the mitral valve. Fish mouth stenosis is a characteristic deformity seen in the mitral valve, resulting from the fusion of commissures due to rheumatic inflammation.

**Why the Correct Answer is Right**
In RHD, the immune response to Group A beta-hemolytic streptococcal infection triggers an autoimmune reaction against cardiac tissues. This leads to the deposition of immune complexes and the activation of complement, resulting in inflammation and scarring of the valve leaflets. The commissures, which are the areas where the valve leaflets meet, become fused, leading to the characteristic 'fish mouth' deformity. This fusion restricts the opening of the valve, causing mitral stenosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not provide a clear mechanism related to rheumatic heart disease.
**Option B:** While calcification can occur in mitral stenosis, it is not the primary mechanism responsible for fish mouth stenosis in RHD.
**Option C:** This option mentions fibrosis, but it is not the specific mechanism leading to commissural fusion in fish mouth stenosis.

**Clinical Pearl / High-Yield Fact**
In RHD, the mitral valve is the most commonly affected valve, and the disease often presents with a combination of stenosis and regurgitation. The 'fish mouth' deformity is a classic sign of rheumatic mitral stenosis.

**Correct Answer:** D.