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
A patient has prosthetic valve replacement and he develops endocarditis 8 months later. Organism responsible is - 
Select and explain. 
A. Staph. aureus, B. Strept. viridans, C. Staph. epidermidis, D. HACEK
C: Staph. epidermidis
Answer: C: Staph. epidermidis 
Explanation: Most common cause of prosthetic valve endocarditis upto 12 months is coagulase-negative staphylococci (staph. epidermidis).