<prompt> An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?Staphylococcus aureusStreptococcus gallolyticusPseudomonas aeruginosaStreptococcus pyogens  <response>: 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,Harrisons Principles of Internal Medicine, 18e. 