A 35-year-old man presents with acute onset of hot, swollen right knee with fever of 38.8°C. He reports recent unprotected sexual encounters. Joint aspiration yields purulent fluid with a leukocyte count of 85,000/mm³ with 95% neutrophils. Gram stain shows no organisms. Which organism is the most likely cause of this septic arthritis?
- A Staphylococcus aureus
- B Streptococcus pneumoniae
- C Neisseria gonorrhoeae ✓
- D Escherichia coli
Correct answer: C. Neisseria gonorrhoeae
Explanation
In a young, sexually active adult with purulent monoarthritis and a negative Gram stain, Neisseria gonorrhoeae is the most likely organism. Gonococcal arthritis often presents with migratory polyarthralgia, tenosynovitis, and dermatitis, but may present as frank septic arthritis. The Gram stain is negative in over 50% of gonococcal joint infections. S. aureus is the most common cause overall but is usually Gram-positive on stain.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.