In intravenous drug users, which organism most commonly causes acute bacterial endocarditis?

• A. Streptococcus viridans
• B. Staphylococcus aureus
• C. Enterococcus
• D. Pseudomonas aeruginosa

Answer: (B)

Staphylococcus aureus is the organism most likely to cause acute bacterial endocarditis in intravenous drug users because it is highly virulent and readily introduced into the bloodstream through nonsterile injections. This rapid-infection organism leads to quick onset of symptoms and tends to damage valves severely, often affecting the right side of the heart (tricuspid valve) since the bloodstream from venous injection first reaches the right heart.

In contrast, viridans streptococci are more associated with subacute endocarditis on abnormal valves after dental work, enterococci with GI/GU tract issues or healthcare exposures, and Pseudomonas can occur but is less commonly the primary cause of acute endocarditis in this population. Thus, the best answer is Staphylococcus aureus.