## **Core Concept**
The question tests knowledge of prosthetic valve endocarditis (PVE) and the timing of infection, which correlates with likely causative organisms. Early PVE (within 2 months of surgery) is commonly caused by **Staphylococcus epidermidis** and other coagulase-negative staphylococci, while late PVE (beyond 2 months) has a broader range of pathogens.

## **Why the Correct Answer is Right**
The patient developed endocarditis eight months after valve replacement, placing this in the category of late prosthetic valve endocarditis. In late PVE, the spectrum of causative organisms shifts towards those more commonly seen in native valve endocarditis, including **Streptococci**, **Enterococci**, and **Staphylococci**. Among these, **Streptococcus viridans** (a type of viridans streptococci) is a common cause of endocarditis, particularly in patients with pre-existing heart conditions or prosthetic valves.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Staphylococcus epidermidis* is more commonly associated with early prosthetic valve endocarditis (within 2 months of surgery), making it less likely in this scenario.
- **Option B:** *Candida* species can cause PVE but are more commonly associated with intravenous drug use, long-term antibiotic use, or in the context of prosthetic valve implantation in a patient with a history of fungal infection.
- **Option C:** *Pseudomonas aeruginosa* can cause endocarditis, especially in intravenous drug users, but it is not the most common cause of late prosthetic valve endocarditis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the timing of prosthetic valve endocarditis can give clues about the likely causative organism: early PVE (0-2 months post-surgery) is often due to **coagulase-negative staphylococci** like *Staphylococcus epidermidis*, while late PVE (>2 months) has a microbiological profile similar to native valve endocarditis, with **Streptococcus viridans** being a significant pathogen.

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