Which organism accounts for more than 60% of infective endocarditis cases in intravenous drug abusers?

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

Answer: (A)

Direct inoculation of skin flora into the bloodstream during intravenous injection makes Staphylococcus aureus the most common cause of infective endocarditis in this group. Its high virulence leads to rapid, aggressive infection that often involves the right-sided heart valves (most commonly the tricuspid valve) in IV drug users. The other organisms listed—viridans streptococci from oral sources, enterococci from GI/GU exposures, and Haemophilus parainfluenzae from the HACEK group—are more associated with subacute endocarditis in patients with dental issues or healthcare exposures and are less likely to account for the majority of cases in this population. Therefore, Staphylococcus aureus accounts for the majority, exceeding 60%, in intravenous drug users.