An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?

Prosthetic valve endocarditis is most commonly caused by Staphylococcus aureus. Many of these strains are resistant to methicillin. It usually involves the tricuspid valve. Prosthetic valve endocarditis usually occur within 2 months of valve surgery. It is usually due to an intraoperative contamination of prosthesis or due to a bacteremic postoperative complication. This type is usually caused by S. aureus, Coagulase-negative staphylococci, facultative gram-negative bacilli, diphtheroids, and fungi. Ref: Karchmer A.W. (2012). Chapter 124. Infective Endocarditis. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds),Harrison's Principles of Internal Medicine, 18e.