A 32-year-old man presents to the emergency department with a 3-day history of severe left knee pain, swelling, and inability to bear weight. He reports a puncture wound to the sole of his foot 10 days ago while walking barefoot on a beach. On examination, the left knee is warm, erythematous, and grossly swollen with severe pain on any movement. Temperature is 38.5°C. Knee aspiration yields 65,000 WBC/μL (92% neutrophils), glucose 18 mg/dL (serum glucose 120 mg/dL), protein 6.2 g/dL, and Gram stain shows Gram-negative rods. Blood cultures are pending. Which organism is most likely responsible for this septic arthritis?

Explanation

## Clinical Presentation & Key Features **High-Yield:** This patient has a classic presentation of septic arthritis secondary to a puncture wound — a high-risk scenario for *Pseudomonas aeruginosa* infection. ### Epidemiological Clue: Puncture Wound Pseudomonas is the most common pathogen in septic arthritis following penetrating trauma, particularly: - Puncture wounds through rubber-soled shoes (contaminates with environmental Pseudomonas) - Puncture wounds to the foot or lower extremity - Marine or aquatic exposures **Key Point:** Pseudomonas aeruginosa is a Gram-negative rod found in soil and water, and is the organism of choice for post-puncture-wound septic arthritis [cite:Tuli's Orthopedics 6e Ch 8]. ### Synovial Fluid Analysis The aspirate shows: - WBC count 65,000/μL (consistent with bacterial infection; typically >50,000 in septic arthritis) - 92% neutrophils (acute bacterial pattern) - Low glucose (18 mg/dL vs serum 120 = low synovial:serum ratio <0.5, typical of bacterial infection) - High protein (6.2 g/dL, consistent with inflammation) - **Gram-negative rods** — narrows to Gram-negative organisms ### Why Pseudomonas Over Other Gram-Negatives While other Gram-negative organisms can cause septic arthritis, Pseudomonas is the epidemiological match for: - Puncture wound mechanism - Environmental / aquatic exposure - Foot trauma ## Differential Considerations | Organism | Common Portal | Typical Setting | Gram Stain | | --- | --- | --- | --- | | *Pseudomonas aeruginosa* | Puncture wound, environmental | Foot trauma, water exposure | Gram-negative rod | | *Staphylococcus aureus* | Hematogenous, direct inoculation | Post-surgical, skin infection | Gram-positive coccus | | *Streptococcus pyogenes* | Hematogenous | Skin/soft tissue infection | Gram-positive coccus | | *Neisseria gonorrhoeae* | Hematogenous (sexually transmitted) | Disseminated gonococcal infection | Gram-negative diplococcus | **Clinical Pearl:** The combination of puncture wound + Gram-negative rod + foot location = Pseudomonas until proven otherwise. This is a high-yield association for NEET PG. ## Management Implications 1. **Empiric coverage** for post-puncture-wound septic arthritis should include anti-Pseudomonas agents (fluoroquinolone or anti-Pseudomonas beta-lactam) 2. **Surgical debridement** is mandatory given the penetrating trauma 3. **Repeat aspiration** and culture confirmation essential before narrowing therapy