Staphylococcus aureus (Answer: D)
Staphylococcus aureus is definitively the most common microbial cause of infective endocarditis in intravenous drug users. This is consistently documented across multiple high-quality guidelines and represents a critical clinical distinction from non-drug user populations.
Evidence from Guidelines
The 2009 ESC Guidelines explicitly state that "the main predisposing factor for S. aureus IE may be intravenous drug abuse" 1. The 2008 ACC/AHA Guidelines are even more specific, noting that "the latter species [staphylococcal organisms] is also the most frequent organism in endocarditis resulting from intravenous drug abuse" and that "eighty percent of tricuspid valve infection is by Staphylococcus aureus" 2.
Multiple guideline documents consistently identify S. aureus as "the most frequent pathogen" in this population 3, 2. This is particularly important because intravenous drug use is the most common cause of tricuspid valve (right-sided) endocarditis, and S. aureus accounts for approximately 80% of these cases.
Supporting Research Evidence
Recent research confirms this pattern:
- A 2022 Canadian review notes that "a majority [of IDU-IE cases] are a result of Streptococcus aureus" 4
- A 2025 case study identifies S. aureus as the primary pathogen "frequently seen in intravenous drug users" 5
- A 2022 Bronx study found "Staphylococcus aureus was the most common isolated pathogen (45.1% of monomicrobial IE)" 6
- A 2017 retrospective study confirmed "S. aureus is the predominant causative organism" 7
Clinical Implications
Key distinguishing features of IDU-associated endocarditis:
- Right-sided involvement predominates (especially tricuspid valve) - unlike non-drug users where left-sided valves are more commonly affected
- S. aureus accounts for ~80% of tricuspid valve infections in this population
- MRSA represents approximately half of S. aureus cases in some urban centers 6
- Higher rates of Gram-negative and fungal infections compared to non-drug users, though still much less common than S. aureus 8
Why Not the Other Options?
- Viridans streptococci (A): While historically the most common cause of IE overall, they are NOT predominant in IV drug users. They are more associated with dental procedures and native valve disease in non-drug users
- Pseudomonas (B): Uncommon cause of IE even in drug users
- Staphylococcus epidermidis (C): More commonly associated with prosthetic valve endocarditis, particularly early prosthetic valve endocarditis (<2 months post-surgery) 2
- Enterococcus faecalis (E): Associated with genitourinary/gastrointestinal manipulation and malignancy, not primarily with IV drug use 2
The answer is unequivocally D) Staphylococcus aureus, supported by consistent guideline-level evidence and confirmed by contemporary research across multiple geographic regions.