A 45-year-old woman presents with an acutely painful, swollen, warm right knee for 2 days with fever. She is sexually active and has a recent history of urethritis. Joint aspiration yields purulent fluid with WBC 90,000/µL (95% neutrophils). Gram stain shows no organisms. Which organism is most likely responsible?
- A Staphylococcus aureus
- B Neisseria gonorrhoeae ✓
- C Streptococcus pyogenes
- D Pseudomonas aeruginosa
Correct answer: B. Neisseria gonorrhoeae
Explanation
In a sexually active adult with concomitant urethritis and septic arthritis with negative Gram stain despite high WBC count, Neisseria gonorrhoeae is the most likely organism. Gonococcal arthritis is the most common cause of septic arthritis in young sexually active adults. Staphylococcus aureus remains the overall most common cause of septic arthritis in adults, but the clinical context with urethritis shifts the probability to gonococcus.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.