Acute endocarditis is most commonly caused by which organism?

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

Answer: (C)

Acute endocarditis is driven by highly virulent organisms that rapidly invade and destroy heart valves, leading to a sudden, severe clinical picture. The organism that most commonly causes this rapid, destructive infection is Staphylococcus aureus, which readily adheres to normal valves and produces aggressive tissue invasion and septic manifestations. This contrasts with viridans group streptococci, which typically cause a subacute, slower-onset infection on abnormal valves after dental work. Enterococcus faecalis is more often linked to GI or GU procedures and nosocomial settings, and Pseudomonas aeruginosa is more characteristic of infections in IV drug users or prosthetic material but is less common overall for acute endocarditis. Hence, Staphylococcus aureus is the best answer.