**Core Concept**
Prosthetic valve endocarditis (PVE) is a devastating complication of heart valve replacement surgery, caused by bacterial or fungal infections. In the early postoperative period, certain organisms are more likely to cause PVE due to their virulence and ability to colonize prosthetic material.

**Why the Correct Answer is Right**
The correct answer is **Staphylococcus epidermidis**, a coagulase-negative staphylococcus that is commonly found on the skin and mucous membranes. **S. epidermidis** is a leading cause of PVE within 60 days of surgery due to its ability to form biofilms on prosthetic valve material, which provides a protective environment for the bacteria to grow and evade the host's immune response. This organism is particularly virulent in the early postoperative period, when the immune system is suppressed and the prosthetic valve is still susceptible to colonization.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus aureus is a common cause of endocarditis, but it is more likely to cause PVE after 60 days of surgery. **Option B:** Streptococcus viridans is a normal oral flora that can cause endocarditis, but it is less likely to cause PVE within 60 days of surgery. **Option C:** Pseudomonas aeruginosa is a opportunistic pathogen that can cause endocarditis, but it is not typically associated with PVE within 60 days of surgery.

**Clinical Pearl / High-Yield Fact**
The timing of PVE is crucial in determining the causative organism. Early PVE (within 60 days) is typically caused by **S. epidermidis**, while late PVE (after 60 days) is more likely to be caused by other organisms, including **S. aureus** and Streptococcus viridans.

**Correct Answer:** C. Staphylococcus epidermidis