Which organism is the most common cause of endocarditis in intravenous drug users and is a Gram-positive cocci in clusters?

• A. Staphylococcus aureus
• B. Streptococcus viridans
• C. Enterococcus faecalis
• D. Staphylococcus epidermidis

Answer: (A)

In intravenous drug users, the most common cause of infective endocarditis is Staphylococcus aureus. This organism is a Gram-positive coccus that characteristically grows in grape-like clusters, which matches the clue in the stem. IV drug use introduces skin flora directly into the bloodstream, and S. aureus is highly virulent and capable of rapid, acute infection, often affecting the tricuspid valve on the right side of the heart.

Other organisms listed are associated with endocarditis in different contexts: Streptococcus viridans typically causes subacute endocarditis after dental work and appears as Gram-positive cocci in chains; Enterococcus faecalis is another cause in older or hospitalized patients and appears in pairs or short chains; Staphylococcus epidermidis is a coagulase-negative Staphylococcus linked more to prosthetic devices and hospital-acquired infections. The combination of IV drug use and Gram-positive cocci in clusters points to Staphylococcus aureus as the best answer.