In intravenous drug users, which organism accounts for the majority of infective endocarditis cases?

• A. Haemophilus parainfluenzae
• B. Enterococci
• C. Staphylococcus aureus
• D. Viridans streptococci

Answer: (C)

Staphylococcus aureus is most common in intravenous drug users because skin flora gain direct access to the bloodstream through injection. This allows S. aureus to seed the heart rapidly, usually affecting the tricuspid valve on the right side, and to cause an acute, destructive endocarditis. Its virulence enables quick tissue invasion and high fever, with a risk of septic pulmonary emboli from right-sided lesions. Other organisms—like viridans streptococci from dental sources, enterococci from GI/GU sources or healthcare exposure, and HACEK organisms such as Haemophilus parainfluenzae—tend to cause endocarditis in different settings (subacute courses on abnormal valves or with distinct source exposures) and are less associated with IV drug use.