**Core Concept**
The question is testing the understanding of the pathophysiology of rheumatic heart disease (RHD) and its complications during pregnancy. RHD is a chronic inflammatory disease that can lead to valve damage, particularly of the mitral valve, and may result in heart failure. Pregnancy can exacerbate heart failure due to increased cardiac output and volume load.

**Why the Correct Answer is Right**
In a pregnant patient with RHD, failure does not occur if she has a _tricuspid_ valve involvement. This is because the right heart has a lower pressure system and is less affected by the increased volume load of pregnancy. The tricuspid valve is also less likely to be damaged by the inflammatory process of RHD. In contrast, mitral valve involvement is more common and can lead to significant left-sided heart failure during pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mitral valve involvement is a common complication of RHD and can lead to left-sided heart failure during pregnancy.
* **Option B:** Aortic valve involvement can also lead to significant heart failure during pregnancy due to the increased pressure load on the left ventricle.
* **Option C:** Pulmonary valve involvement is rare in RHD and is not typically associated with heart failure during pregnancy.

**Clinical Pearl / High-Yield Fact**
In pregnant patients with RHD, the risk of heart failure is highest when there is mitral valve involvement and significant left-sided disease. A thorough cardiac evaluation and monitoring are essential to prevent complications during pregnancy.

**Correct Answer: C. Tricuspid valve involvement.**