**Core Concept**
Infective endocarditis (IE) is an infection of the heart valves or the endocardium, typically caused by bacteria or fungi. The risk of developing IE is higher in individuals with pre-existing cardiac lesions that alter the normal blood flow or create areas of turbulent flow, making it easier for pathogens to adhere and colonize.

**Why the Correct Answer is Right**
The cardiac lesions at highest risk of IE are those with significant valvular abnormalities, particularly those with **prosthetic valves**. Prosthetic valves create a non-endothelialized surface, providing a conducive environment for bacterial colonization and subsequent IE. Additionally, the presence of a prosthetic valve disrupts the normal endothelial layer, increasing the risk of bacterial adherence and multiplication.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mitral valve prolapse is a condition where the mitral valve leaflets bulge back into the left atrium during systole. While it can increase the risk of IE, it is not as high as that associated with prosthetic valves.
* **Option B:** Atrial septal defects (ASDs) are characterized by an abnormal opening between the atria. While they can increase the risk of IE, the risk is not as high as that associated with prosthetic valves.
* **Option D:** Ventricular septal defects (VSDs) are characterized by an abnormal opening between the ventricles. While they can increase the risk of IE, the risk is not as high as that associated with prosthetic valves.

**Clinical Pearl / High-Yield Fact**
Prosthetic valves, particularly mechanical valves, are at higher risk of IE due to the non-endothelialized surface and the presence of a foreign body, making them a high-risk group for IE. This is especially true for patients with a history of IE or those with certain underlying conditions, such as rheumatic heart disease.

**Correct Answer:** C. Prosthetic valve