Which organism is the most common causative agent of septic arthritis in a previously healthy adult with acute monoarthritis?

Explanation

## Microbiology of Septic Arthritis **Key Point:** Staphylococcus aureus accounts for 40–50% of all septic arthritis cases in immunocompetent adults and is the single most common organism across all age groups and risk categories. ### Organism-Specific Epidemiology | Organism | Frequency | Clinical Context | Key Features | |----------|-----------|------------------|---------------| | *S. aureus* | 40–50% | Most common overall; all ages | Rapidly progressive; high virulence | | *S. pneumoniae* | 10–15% | Elderly, asplenic, immunocompromised | Often polyarticular | | *N. gonorrhoeae* | 5–10% | Sexually active young adults | Migratory polyarthritis initially; monoarthritis late | | Gram-negative bacilli | 5–10% | IVDU, elderly, immunocompromised | E. coli, Klebsiella, Pseudomonas | | *Streptococcus* spp. (Group B, G) | 5–10% | Elderly, diabetes, immunocompromised | — | **High-Yield:** In a previously healthy adult presenting with acute monoarthritis, *S. aureus* is the default answer until proven otherwise. It has the highest propensity to seed large joints (knee > hip > shoulder) and causes the most aggressive, rapidly progressive infections. **Clinical Pearl:** The organism distribution shifts with risk factors—IVDU favors *S. aureus* and gram-negative organisms; sexually active young adults must raise suspicion for *N. gonorrhoeae*; elderly and immunocompromised patients see higher rates of *S. pneumoniae* and gram-negatives. **Mnemonic:** **SANG** — *S. aureus*, *N. gonorrhoeae*, Gram-negatives (the "big three" in septic arthritis; add pneumococcus for immunocompromised). [cite:Harrison 21e Ch 330]