The most probable etiology of bacterial endocarditis involving the tricuspid valve is illicit intravenous drug use, which can introduce skin organisms into the venous system that then attack the tricuspid valve. Staphylococcus aureus accounts for between 60% and 90% of cases of endocarditis in intravenous drug users. The endocarditis associated with congenital hea disease typically involves either damaged valves or atrial or ventricular septal defects. The tricuspid valve is not paicularly vulnerable. Rheumatic fever most commonly damages the mitral and aoic valves, and tricuspid damage is usually less severe and seen only when the mitral and aoic valves are heavily involved. Consequently, secondary bacterial endocarditis involving only the tricuspid valve in a patient with a history of rheumatic fever would be unusual. Rheumatoid ahritis is not associated with bacterial endocarditis. Ref: Brooks G.F. (2013). Chapter 14. The Streptococci, Enterococci, and Related Genera. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.