Final answer:
Intravenous drug users are most commonly affected by Staphylococcus aureus endocarditis, with the tricuspid valve being the most involved, leading to a regurgitation murmur. Acute endocarditis operates rapidly with severe valve damage, while subacute progresses over months with slow damage and potential need for valve replacement.
Step-by-step explanation:
The most common cause of infective endocarditis in intravenous drug users (IVDU) is the bacterium Staphylococcus aureus. The valve most commonly affected in IVDU-associated endocarditis is the tricuspid valve. Infective endocarditis can lead to various murmur features, depending on the valve affected and the extent of damage. Tricuspid valve infection in IVDU typically results in a tricuspid regurgitation murmur, which is best heard at the lower left sternal border and may increase with inspiration (Carvallo's sign).
Compare acute and subacute bacterial endocarditis, both require high doses of intravenous antibiotics, but acute endocarditis presents more rapidly and is usually caused by highly virulent organisms, leading to a quick destruction of heart valves. In contrast, subacute bacterial endocarditis progresses more slowly and can manifest signs such as fever and heart valve damage over months, potentially requiring valve replacement due to fibrosis caused by the immune response.
In carditis, various valvular disorders can occur, with mitral regurgitation being the most common overall, but in the context of IVDU, the tricuspid valve is most likely to be involved due to its exposure to venous blood which may carry bacteria introduced by needle use.