A 68-year-old male undergoes total knee replacement. Six weeks postoperatively he develops sudden severe knee pain, fever 38.8°C, and a swollen, warm joint. Joint aspiration reveals 78,000 WBC/μL with 92% PMNs, gram stain negative. Which organism is most likely to grow on culture?

• A Staphylococcus aureus ✓
• B Coagulase-negative Staphylococcus (e.g., S. epidermidis)
• C Streptococcus agalactiae
• D Cutibacterium acnes

Explanation

Early acute periprosthetic joint infection (within 3 months) presenting with high fever, rapid onset, and markedly elevated synovial WBC is most commonly caused by virulent organisms, with Staphylococcus aureus being the single most common pathogen accounting for ~35% of early TKR infections. Coagulase-negative staphylococci dominate late and delayed (2-24 months) low-grade infections. Cutibacterium acnes is predominantly a shoulder arthroplasty pathogen. The very high WBC with predominantly PMNs confirms acute septic process.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.