A 42-year-old woman with rheumatoid arthritis on methotrexate presents with acute monoarticular knee effusion. Synovial fluid culture grows Staphylococcus aureus. Regarding the microbiology and risk factors of septic arthritis, all of the following are true EXCEPT:

Explanation

## Microbiology of Septic Arthritis ### Causative Organisms — Epidemiology **Key Point:** The distribution of organisms in septic arthritis varies by patient population and risk factors: | Organism | Frequency | Risk Factors | Notes | | --- | --- | --- | --- | | **Staphylococcus aureus** | 50–60% | All ages; post-arthroscopy; RA | Most common overall | | **Streptococcus pyogenes** | 10–15% | Immunocompromised; elderly | Less common | | **Streptococcus pneumoniae** | 5–10% | Elderly; asplenic patients | NOT the most common | | **Gram-negative bacilli** | 10–20% | IVDU; immunocompromised; elderly | E. coli, Pseudomonas | | **Neisseria gonorrhoeae** | 5–10% | Sexually active; young adults | Migratory polyarthritis | | **Mycobacterium tuberculosis** | 1–5% | Endemic areas; immunocompromised | Chronic monoarthritis | **High-Yield:** **Staphylococcus aureus is the MOST COMMON organism in septic arthritis across all age groups and populations**, accounting for 50–60% of cases. Streptococcus pneumoniae is NOT the most common — it accounts for only 5–10% of cases. ### Age-Related and Risk Factor Patterns **Key Point:** Organism distribution shifts with patient characteristics: 1. **Neonates (<3 months):** Group B Streptococcus, Gram-negative bacilli, S. aureus 2. **Children (3 months–5 years):** S. aureus, H. influenzae (now rare post-vaccination), S. pyogenes 3. **Adults:** S. aureus (50–60%), Streptococcus spp., Gram-negatives 4. **Elderly:** S. aureus, Streptococcus spp., Gram-negatives, Mycobacterium tuberculosis 5. **IVDU:** S. aureus, Gram-negatives (Pseudomonas), Candida 6. **Immunocompromised:** Gram-negatives, Mycobacterium, fungi ### Gonococcal Arthritis **Clinical Pearl:** Neisseria gonorrhoeae causes a distinctive **migratory polyarthritis** with: - Migratory pattern (moves from joint to joint) - Pustular or vesicular rash (often on trunk, extremities) - Often preceded by urethritis or cervicitis - More common in young, sexually active individuals - This presentation is TRUE and a classic NEET PG fact. ### Gram-Negative Organisms **Key Point:** Gram-negative bacilli (E. coli, Pseudomonas, Klebsiella) are overrepresented in: - IV drug users (especially Pseudomonas aeruginosa — can seed unusual joints like sternoclavicular, sacroiliac) - Immunocompromised patients (HIV, malignancy, chronic corticosteroids) - Elderly patients - Patients with recent urinary instrumentation This option is TRUE. ### Why This Patient Has S. aureus **Clinical Pearl:** This 42-year-old with rheumatoid arthritis on methotrexate is immunocompromised, yet still developed S. aureus septic arthritis — consistent with its dominance as the #1 organism even in RA patients. RA itself is a risk factor for septic arthritis.