A 35-year-old man presents with acute onset swelling, warmth, and severe pain in the right knee. Synovial fluid analysis shows WBC 85,000/μL with 92% neutrophils, positive Gram stain, and culture pending. What is the most common causative organism in septic arthritis of the knee in an immunocompetent adult in India?

Explanation

## Most Common Organism in Septic Arthritis **Key Point:** Staphylococcus aureus accounts for 40–50% of all septic arthritis cases in immunocompetent adults worldwide, including India. ### Epidemiology of Septic Arthritis Organisms | Organism | Frequency | Clinical Context | Notes | |----------|-----------|------------------|-------| | **Staphylococcus aureus** | 40–50% | Immunocompetent, post-traumatic, hematogenous | Most common; MRSA rising in nosocomial cases | | Streptococcus pyogenes | 10–15% | Post-streptococcal, immunocompromised | Less common than S. aureus | | Neisseria gonorrhoeae | 5–10% | Sexually active, disseminated gonococcemia | Rare in India; more common in Western populations | | Gram-negative bacilli | 5–10% | Immunocompromised, IV drug users, elderly | E. coli, Klebsiella; less common in acute monoarthritis | ### Why S. aureus Dominates 1. **Virulence factors:** Produces multiple adhesins (fibronectin-binding proteins, collagen-binding proteins) enabling joint invasion 2. **Hematogenous seeding:** High propensity for bacteremia from skin/soft tissue sources 3. **Pathogenesis:** Rapidly causes synovial inflammation, cartilage damage, and joint destruction if untreated **High-Yield:** In any patient with acute monoarthritis + fever + elevated inflammatory markers, empiric therapy must cover S. aureus (including MRSA in nosocomial settings) until culture results return. **Clinical Pearl:** The knee is the single most common site of septic arthritis (50% of cases), and S. aureus is the leading cause at this site in both children and adults. **Mnemonic:** **SAG** — **S**taphylococcus **A**ureus is the **G**old standard organism in septic arthritis.