What is the most likely cause of surgical site infection in a patient with a previous surgical scar on their hand, presenting with pain, redness, swelling, and greenish discharge?

Most Likely Causative Organism of Surgical Site Infection

The most likely cause of this surgical site infection is Staphylococcus aureus (Answer A), as it is the predominant pathogen in clean surgical procedures involving the hand, where the infection arises from the patient's skin flora. 1, 2

Pathogen Distribution in Surgical Site Infections

Clean Procedures (Hand Surgery)

• Staphylococcus aureus from the patient's skin flora is the most common cause of surgical site infection in clean surgical procedures where the gastrointestinal, gynecologic, and respiratory tracts have not been entered 1, 2
• S. aureus accounts for the majority of SSIs in trunk and extremity surgeries away from the axilla or perineum 2
• The patient's own skin flora is the predominant source of infection in these cases 2

Clinical Presentation Supporting S. aureus

• The presence of purulent discharge (described as "greenish" in this case) is diagnostic of SSI and consistent with staphylococcal infection 1, 3
• Pain, redness, and swelling are classic signs of superficial incisional SSI, which most commonly involves S. aureus 1
• SSIs rarely occur in the first 48 hours; infections presenting after this period are typically due to S. aureus rather than the early-onset pathogens 1

Why Other Options Are Less Likely

Streptococcus pyogenes (Option B)

• S. pyogenes causes SSIs almost exclusively within the first 48 hours after surgery, presenting with rapid onset and dramatic clinical findings 1
• Late-presenting infections (beyond 48 hours) are much less likely to be streptococcal 1

Clostridium perfringens (Option C)

• Clostridial infections occur within the first 48 hours postoperatively with severe systemic toxicity 1
• These infections typically show minimal white blood cells on Gram stain and have a fulminant course inconsistent with the described presentation 1

Pseudomonas aeruginosa (Option D)

• While Pseudomonas can produce greenish discharge, it is not the most common SSI pathogen in clean hand surgery 1
• Gram-negative bacteria like Pseudomonas are more commonly isolated from operations involving the axilla, gastrointestinal tract, perineum, or female genital tract 1
• In specific anatomical regions like the groin, enteric flora including gram-negatives may colonize skin, but this does not apply to hand surgery 1

Clinical Management Implications

Antibiotic Selection

• First-generation cephalosporin (such as cefazolin) or antistaphylococcal penicillin should be used for methicillin-susceptible S. aureus 1, 4
• If risk factors for MRSA are present (nasal colonization, prior MRSA infection, recent hospitalization, recent antibiotics), vancomycin, linezolid, daptomycin, telavancin, or ceftaroline should be considered 1

Primary Treatment

• The most important therapy is to open the incision, evacuate infected material, and continue dressing changes until the wound heals by secondary intention 1
• Antibiotics are recommended only if systemic inflammatory response criteria are present, signs of organ failure exist, or the patient is immunocompromised 1
• If there is <5 cm of erythema and induration with minimal systemic signs (temperature <38.5°C, WBC <12,000 cells/µL, pulse <100 beats/minute), antibiotics may be unnecessary after drainage 1