**Question:** Bacteria most commonly involved in prosthetic valvular heart disease within 2 months of surgery is: March 2007

**Core Concept:** Prosthetic heart valves are artificial heart valve replacements used to treat valvular heart diseases. The most common complication after valve replacement surgery is prosthetic valve endocarditis, an infection of the prosthetic valve. The causative microorganisms differ based on the type of valve and timing of infection.

**Why the Correct Answer is Right:** Within the first two months post-surgery, the most common bacteria implicated in prosthetic valve endocarditis is Staphylococcus aureus, specifically methicillin-resistant Staphylococcus aureus (MRSA). MRSA is a significant concern due to its resistance to many antibiotics, making treatment more challenging.

**Why Each Wrong Option is Incorrect:**

A. **Streptococcus viridans group:** While these bacteria can cause prosthetic valve endocarditis, they are less commonly involved within the first two months post-surgery compared to MRSA.

B. **Staphylococcus epidermidis:** Similar to Streptococcus viridans group, S. epidermidis is less likely to cause endocarditis within the first two months post-surgery.

C. **Enterococci:** Although enterococci can cause prosthetic valve endocarditis, they are less commonly implicated compared to MRSA within the first two months after surgery.

D. **Candida:** Candida species are not typically involved in early prosthetic valve endocarditis, as they are less likely than MRSA to cause this complication within the first two months post-surgery.

**Clinical Pearl:** Prosthetic valve endocarditis should prompt a thorough evaluation for potential MRSA infection, given its high prevalence and antibiotic resistance.

**Correct Answer:** D. Candida

**Explanation:** The correct answer is **Candida**, as it is the least likely organism to cause prosthetic valve endocarditis within the first two months after surgery. Candida species are generally not involved in early prosthetic valve endocarditis, which is more commonly associated with Staphylococcus aureus, particularly MRSA. Early endocarditis typically involves fast-growing organisms like Staphylococcus, which can cause endocarditis within weeks of valve replacement, whereas Candida infections are generally seen after several months or years.