**Core Concept**
Carcinoid syndrome of the heart is a condition where a carcinoid tumor releases serotonin and other vasoactive substances, leading to fibrosis and valve damage. The most common valvular lesion associated with carcinoid syndrome is due to the unique pharmacological effects of serotonin on the heart valves.

**Why the Correct Answer is Right**
Serotonin binds to 5-HT2 receptors on the valve endothelium, leading to proliferation of fibroblasts and subsequent fibrosis. This results in thickening and stiffening of the valve leaflets, ultimately causing regurgitation. The tricuspid valve is more commonly affected due to the right-to-left shunt of serotonin from the systemic circulation to the pulmonary circulation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mitral valve is less commonly involved in carcinoid syndrome.
* **Option B:** Aortic valve is not typically affected by carcinoid syndrome.
* **Option C:** Pulmonary valve is not a common site for carcinoid-induced valve lesions.

**Clinical Pearl / High-Yield Fact**
Carcinoid syndrome is often associated with a characteristic facial flushing, diarrhea, wheezing, and heart valve lesions. A high index of suspicion is necessary for diagnosis, as symptoms can be nonspecific and may be attributed to other conditions.

**Correct Answer:** C. Tricuspid valve