## **Core Concept**
Late prosthetic valve endocarditis (PVE) typically occurs more than 60 days after valve implantation. It is often caused by organisms that are part of the normal flora of the skin or mucous membranes, which can colonize the prosthetic valve.

## **Why the Correct Answer is Right**
The correct answer, **D. Streptococcus viridans**, is a common cause of late PVE. *Streptococcus viridans* is part of the normal oral flora and can cause endocarditis, particularly after dental procedures that lead to bacteremia. This organism is known for its role in subacute bacterial endocarditis and is a leading cause of late PVE due to its ability to adhere to prosthetic surfaces.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Staphylococcus aureus* is more commonly associated with early PVE (within 60 days of surgery) and is often related to perioperative contamination. While it can cause late PVE, it's less typical compared to *Streptococcus viridans*.
- **Option B:** *Coagulase-negative staphylococci* are commonly implicated in early PVE, usually within 60 days post-surgery, and are often related to perioperative contamination of the prosthetic valve.
- **Option C:** *Enterococci* can cause PVE but are less commonly associated with late PVE compared to *Streptococcus viridans*. They are more often seen in cases with underlying gastrointestinal or genitourinary conditions.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that late PVE is often associated with organisms like *Streptococcus viridans*, *Enterococci*, and *Staphylococci*, but *Streptococcus viridans* is particularly notable for its role in cases following dental procedures. Patients with prosthetic valves should receive antibiotic prophylaxis before dental procedures to reduce the risk of PVE.

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