This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Bacteria most commonly involved in prosthetic valvular hea disease within 2 months of surgery is: March 2007 
Select and explain. 
A. Staphylococcus epidermidis, B. Enterococci, C. Streptococcus viridans, D. Hemophilus
A: Staphylococcus epidermidis
Answer: A: Staphylococcus epidermidis 
Explanation: Ans. A: Staphylococcus epidermidis Staphylococci account for 20 to 30% of all cases of endocarditis in the elderly. The predominant species, Staphylococcus aureus, often causes nosocomial endocarditis. S. epidermidis is isolated in < 5% of patients with native valve endocarditis, but it is the most common cause of cases that involve prosthetic valves occurring early after surgery in elderly and in younger patients. Gram-negative aerobic bacilli and fungi cause 2 to 3% of all cases of endocarditis. Infections due to Bacteroides species and mixed infections are rare among the elderly.