In intravenous drug users, which heart valve is most commonly infected in bacterial endocarditis?

• A. Mitral
• B. Aortic
• C. Tricuspid
• D. Pulmonary

Answer: (C)

Bacteria enter the venous system directly with intravenous drug use, so the right side of the heart is affected first. The tricuspid valve is the first heart valve the blood encounters on return from the systemic veins, making it the most likely site for infectious endocarditis in this setting. Vegetations form there and can shed septic emboli to the lungs, leading to pulmonary symptoms such as nodules, infiltrates, or abscesses. Staphylococcus aureus is the typical culprit in IV drug users, and left-sided valves like the mitral or aortic are more commonly involved in those without IV drug use. The pulmonary valve is involved far less often.