## **Core Concept**
Native valve endocarditis refers to the infection of a heart valve that is native to the heart, as opposed to a prosthetic valve. The most common causes of native valve endocarditis are typically bacterial, with certain species being more prevalent than others.

## **Why the Correct Answer is Right**
The correct answer, **Streptococcus viridans**, is a group of streptococci that are commonly found in the mouth and are known to cause endocarditis, particularly after dental procedures that lead to bacteremia. This organism is responsible for approximately 30-40% of cases of native valve endocarditis and is considered the most common cause. **Streptococcus viridans** has a particular affinity for damaged heart valves.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Staphylococcus aureus* is a common cause of endocarditis, especially in intravenous drug users and those with healthcare-associated infections, but it is not the most common cause of native valve endocarditis in the general population.
- **Option B:** *Enterococci* can cause endocarditis, particularly in older adults or those with certain underlying health conditions, but they are not as common as **Streptococcus viridans**.
- **Option D:** *Pseudomonas aeruginosa* is more commonly associated with endocarditis in intravenous drug users but is not the most common cause of native valve endocarditis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Streptococcus viridans** endocarditis often follows a subacute course and is associated with a history of dental procedures or other sources of bacteremia. Prevention includes antibiotic prophylaxis before dental procedures in patients with certain heart conditions.

## **Correct Answer:** . Streptococcus viridans