**Core Concept**
Carcinoid syndrome is a condition characterized by the excessive production of serotonin (5-HT) and other vasoactive peptides by neuroendocrine tumors, leading to various systemic effects, including cardiovascular manifestations.

**Why the Correct Answer is Right**
The primary mechanism of carcinoid syndrome-induced valvular disease is the production of serotonin, which causes fibrosis and thickening of the cardiac valve leaflets, particularly the tricuspid and pulmonary valves. The serotonin is metabolized to 5-hydroxyindoleacetic acid (5-HIAA) by the enzyme monoamine oxidase (MAO), but in the context of carcinoid syndrome, MAO levels are often normal or low. Instead, the serotonin is primarily metabolized by the enzyme aldehyde dehydrogenase, which is present in the cardiac valves. This leads to the accumulation of 5-HT metabolites, causing valve fibrosis and thickening. The tricuspid and pulmonary valves are more commonly affected due to the right-sided heart dominance in the production of serotonin.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because carcinoid syndrome primarily affects the tricuspid and pulmonary valves, not the aortic valve.
* **Option B:** This option is incorrect because carcinoid syndrome does not primarily involve the mitral valve.
* **Option D:** This option is incorrect because while carcinoid syndrome can cause right-sided heart failure, it is not the primary mechanism leading to valvular disease.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with carcinoid syndrome, it is essential to consider the presence of valvular disease, particularly tricuspid and pulmonary valve involvement, and to investigate for the presence of serotonin-producing neuroendocrine tumors.

**Correct Answer: C. Tricuspid and pulmonary valves.**