A 65-year-old man with diabetes mellitus presents with acute onset of severe pain, swelling, and erythema of the right knee with fever (39.5°C). Synovial fluid analysis shows 68,000 WBC/μL with 92% neutrophils. Gram stain shows gram-positive cocci in clusters. Which organism is most likely responsible?

• A Neisseria gonorrhoeae
• B Staphylococcus aureus ✓
• C Streptococcus pneumoniae
• D Escherichia coli

Explanation

Staphylococcus aureus is the most common cause of septic arthritis in adults, especially in diabetics and elderly. Synovial WBC greater than 50,000/μL with greater than 90% neutrophils strongly suggests infection. N. gonorrhoeae is seen in young sexually active adults. E. coli is more common in IV drug users.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.