A 28-year-old woman presents to the emergency department with a 3-day history of fever (39.2°C), severe right knee pain, and inability to bear weight. She has no recent trauma or joint surgery. On examination, the right knee is swollen, erythematous, and warm to touch with restricted range of motion. Synovial fluid analysis shows WBC 85,000/μL (95% neutrophils), glucose 18 mg/dL, protein 6.2 g/dL, and Gram stain reveals Gram-positive cocci in clusters. Blood culture is pending. What is the most likely organism and the recommended empirical antibiotic therapy?

Question

Accepted Answer

D. Staphylococcus aureus; ceftriaxone + vancomycin. ## Clinical Diagnosis

**Key Point:** Acute bacterial arthritis with Gram-positive cocci in clusters is Staphylococcus aureus until proven otherwise. The synovial fluid profile (WBC >50,000/μL, low glucose, high protein) is classic for septic arthritis.

## Organism Identification

| Feature | S. aureus | S. pyogenes | N. gonorrhoeae | S. epidermidis |
|---------|-----------|-------------|-----------------|----------------|
| **Gram stain** | Cocci in clusters | Cocci in chains | Gram-negative diplococci | Cocci in clusters |
| **Catalase** | Positive | Negative | Negative | Positive |
| **Coagulase** | Positive | Negative | Negative | Negative |
| **Common in septic arthritis** | Yes (most common) | Rare | Yes (disseminated) | Rare (prosthetic joints) |

**High-Yield:** S. aureus causes ~50% of non-gonococcal acute bacterial arthritis in adults. The cluster morphology on Gram stain is pathognomonic.

## Empirical Antibiotic Therapy

**Clinical Pearl:** In acute bacterial arthritis, empirical coverage must include both MSSA and MRSA until culture/susceptibility results are available.

1. **Ceftriaxone + Vancomycin** is the standard empirical regimen:
   - Ceftriaxone: covers MSSA and most Gram-negatives
   - Vancomycin: covers MRSA and provides beta-lactam allergy alternative

2. **Dosing for joint penetration:**
   - Vancomycin: 15–20 mg/kg IV Q8–12H (target trough 15–20 μg/mL)
   - Ceftriaxone: 2 g IV Q12H

3. **De-escalation:** Once culture/susceptibility is available, switch to targeted therapy (e.g., nafcillin or oxacillin for MSSA; continue vancomycin for MRSA).

**Warning:** Do NOT use nafcillin monotherapy empirically — it does not cover MRSA, and MRSA prevalence in India is rising (15–30% in many centres).

**Mnemonic:** **CVAJ** = **C**eftriaxone + **V**ancomycin for **A**cute **J**oint infection.

## Why This Answer

The combination of Gram-positive clusters, septic synovial fluid, and acute monoarthritis in a young adult without prosthetic hardware or recent surgery points to S. aureus. Ceftriaxone + vancomycin is the guideline-recommended empirical choice pending culture results.

Answer

A. Staphylococcus epidermidis; nafcillin

Answer

B. Streptococcus pyogenes; penicillin G

Answer

C. Neisseria gonorrhoeae; ceftriaxone + azithromycin

Explanation

Clinical Diagnosis

Organism Identification

Empirical Antibiotic Therapy

Why This Answer

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Feature	S. aureus	S. pyogenes	N. gonorrhoeae	S. epidermidis
Gram stain	Cocci in clusters	Cocci in chains	Gram-negative diplococci	Cocci in clusters
Catalase	Positive	Negative	Negative	Positive
Coagulase	Positive	Negative	Negative	Negative
Common in septic arthritis	Yes (most common)	Rare	Yes (disseminated)	Rare (prosthetic joints)