A 62-year-old man with a prosthetic aortic valve develops fevers and malaise. His valve was replaced 5 years ago because of aortic stenosis from a bicuspid valve. He has a systolic ejection murmur but no other abnormalities on examination. Blood cultures are most likely to grow which of the following?

**Core Concept**
Prosthetic valve endocarditis (PVE) is a serious infection of a prosthetic heart valve, often caused by bacteria. The risk of PVE is higher in patients with a history of valve replacement due to pre-existing valve disease.

**Why the Correct Answer is Right**
The patient's history of aortic stenosis from a bicuspid valve and subsequent prosthetic valve replacement increases his risk of developing PVE. The most common pathogens responsible for PVE are Staphylococcus aureus and coagulase-negative staphylococci, which can colonize the prosthetic valve. These organisms are likely to be present in blood cultures, particularly in patients with a prosthetic valve.

**Why Each Wrong Option is Incorrect**
* **Option A:** While enterococci can cause endocarditis, they are less common in PVE. Enterococci are more often associated with native valve endocarditis.
* **Option B:** Candida species can cause fungal endocarditis, but they are not the most common pathogens in PVE. Fungal endocarditis is more often seen in patients with a history of intravenous drug use or immunocompromised states.
* **Option D:** Streptococcus viridans is a common cause of native valve endocarditis, particularly after dental procedures. However, it is less common in PVE.

**Clinical Pearl / High-Yield Fact**
In patients with a prosthetic valve, the most common pathogens causing endocarditis are Staphylococcus aureus and coagulase-negative staphylococci. These organisms can colonize the prosthetic valve and lead to PVE.

**Correct Answer:** C. coagulase-negative staphylococci.

✓ Correct Answer: D. Staphylococcus epidermidis