A 32-year-old man from Delhi presents with a 3-day history of fever (39.2°C), severe right knee pain, and swelling. He has a history of intravenous drug use. On examination, the knee is warm, erythematous, and tender with restricted movement. Synovial fluid analysis shows WBC 45,000/μL (90% neutrophils), protein 6.2 g/dL, glucose 18 mg/dL (serum glucose 110 mg/dL), and Gram stain reveals Gram-positive cocci in clusters. Blood culture is pending. Which organism is most likely responsible, and what is the most appropriate initial antibiotic therapy?

Explanation

## Clinical Diagnosis **Key Point:** The combination of acute septic arthritis in an IVDU with Gram-positive cocci in clusters on Gram stain is pathognomonic for *Staphylococcus aureus* infection. ### Diagnostic Clues | Feature | Finding | Significance | |---------|---------|---------------| | **Presentation** | Acute monoarthritis (knee) | Typical for S. aureus | | **Risk factor** | IVDU | S. aureus is the leading cause of bacteremic joint infection in IVDU | | **Gram stain** | Gram-positive cocci in clusters | Characteristic of S. aureus (clusters vs. chains in Streptococcus) | | **Synovial fluid** | WBC 45,000, low glucose (18 vs. 110) | Suppurative infection; low synovial:serum glucose ratio (<0.5) is typical for S. aureus | | **Fever + systemic signs** | High fever, likely bacteremia | S. aureus causes hematogenous spread | ## Antibiotic Selection **High-Yield:** In a patient with no documented MRSA colonization or healthcare exposure, **oxacillin or nafcillin** (β-lactamase-stable penicillins) are the first-line agents for community-acquired S. aureus septic arthritis. These achieve excellent synovial penetration and are superior to vancomycin for MSSA (methicillin-sensitive S. aureus). **Clinical Pearl:** Vancomycin is reserved for: - Documented or suspected MRSA - β-lactam allergy - Severe renal impairment This patient has no such risk factors, making nafcillin/oxacillin the optimal choice. ## Pathophysiology **Mnemonic: IVDU-SAB** — *Intravenous Drug Users* are at high risk for *Staphylococcus aureus Bacteremia* due to: 1. Direct inoculation of skin flora via contaminated needles 2. Impaired immune response (chronic drug use, malnutrition) 3. Right-sided endocarditis → septic emboli to joints ## Additional Management - **Drainage:** Urgent arthrocentesis and surgical drainage (if loculated or large effusion) - **Duration:** 4 weeks IV antibiotics for uncomplicated septic arthritis - **Imaging:** MRI to assess for osteomyelitis or abscess - **Endocarditis screening:** Echocardiography (IVDU + S. aureus bacteremia = 50% risk of IE) [cite:Harrison 21e Ch 328]