**Core Concept**
The patient is experiencing symptoms suggestive of prosthetic valve endocarditis (PVE), a potentially life-threatening complication of valve replacement surgery. The presence of a new murmur and fever in a patient with a prosthetic valve increases the suspicion for PVE.

**Why the Correct Answer is Right**
The most common causative organisms of PVE are Staphylococcus aureus and coagulase-negative staphylococci (CoNS), such as Staphylococcus epidermidis. These organisms are commonly found on the skin and can contaminate the prosthetic valve during surgery. The virulence of these organisms is often related to their ability to form biofilms on the prosthetic surface, making them resistant to antibiotics and host immune responses.

**Why Each Wrong Option is Incorrect**
* **Option A:** Enterococcus faecalis is a common cause of native valve endocarditis, but it is less commonly associated with PVE.
* **Option B:** Streptococcus viridans is a group of organisms that are commonly associated with subacute endocarditis, particularly in the context of dental procedures. However, they are less commonly associated with PVE.
* **Option D:** Pseudomonas aeruginosa is a gram-negative rod that can cause endocarditis, but it is not as commonly associated with PVE as Staphylococcus aureus and CoNS.

**Clinical Pearl / High-Yield Fact**
PVE is often associated with a poor prognosis, with a high risk of mortality and morbidity. Early diagnosis and treatment are critical, and the use of echocardiography to identify vegetations on the prosthetic valve is essential.

**Correct Answer: C. Staphylococcus epidermidis**