**Core Concept**
Endocarditis, particularly infective endocarditis, is a serious complication of intravenous (IV) drug use. The condition occurs when bacteria or other pathogens colonize the heart valves, leading to inflammation and damage. In IV drug users, the most common valve affected is the tricuspid valve.

**Why the Correct Answer is Right**
The tricuspid valve is more susceptible to endocarditis due to its anatomical location and the direction of blood flow. The right heart, which includes the tricuspid valve, receives deoxygenated blood from the body, which is more likely to harbor bacteria. Additionally, the tricuspid valve is less commonly affected by congenital abnormalities or other conditions that may increase its risk for endocarditis. The tricuspid valve's unique properties make it more prone to bacterial colonization and subsequent infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** The mitral valve is more commonly affected in conditions such as rheumatic heart disease and is less likely to be involved in IV drug-related endocarditis.
* **Option B:** The pulmonary valve is rarely affected in endocarditis due to its location and the low pressure and flow through the right ventricle.
* **Option D:** The aortic valve is more commonly affected in conditions such as rheumatic heart disease and is less likely to be involved in IV drug-related endocarditis.

**Clinical Pearl / High-Yield Fact**
IV drug users are at increased risk for tricuspid valve endocarditis due to the direct injection of bacteria into the right heart through the tricuspid valve. This can lead to severe complications, including heart failure and sepsis.

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