Staphylococcus aureus is the Most Common Cause of Surgical Site Infections
The correct answer is A. Staphylococcus aureus, which is the most common pathogen causing surgical site infections, particularly in clean surgical procedures where it originates from the patient's skin flora or the exogenous environment. 1, 2
Epidemiology and Pathogen Distribution
In clean surgical procedures (where no body cavity is entered), S. aureus from the patient's skin flora is the predominant causative organism of SSIs. 1, 2 This accounts for the majority of infections in procedures involving the trunk and extremities away from the axilla or perineum. 2
The clinical presentation described—redness, swelling, purulent discharge, localized pain, and mild fever—is classic for a superficial or deep incisional SSI, which most commonly occurs after 48 hours post-operatively. 1 S. aureus is responsible for approximately 37-45% of all SSIs in community and tertiary care hospitals. 3, 4
Other Pathogens in Context
While the other organisms listed can cause SSIs, they are significantly less common in typical post-operative wound infections:
Streptococcus pyogenes (Option B) causes early SSIs (within 48 hours) with distinctive features including wound drainage with organisms visible on Gram stain but often without white blood cells. 1 This represents a rare but severe presentation requiring urgent intervention. 1
Clostridium perfringens (Option C) also causes early post-operative infections (within 48 hours) and presents with necrotizing features, making it a rare cause of typical SSIs. 1
Pseudomonas aeruginosa (Option D) accounts for only 6.7% of SSIs and is more commonly associated with contaminated procedures or specific anatomical sites. 3
Clinical Implications for Management
The predominance of S. aureus should guide empiric antibiotic selection when systemic therapy is indicated. 1 First-generation cephalosporins (such as cefazolin) or antistaphylococcal penicillins are recommended for methicillin-susceptible S. aureus (MSSA). 1, 5
If risk factors for MRSA are present—including nasal colonization, prior MRSA infection, recent hospitalization, or recent antibiotic use—vancomycin, linezolid, daptomycin, telavancin, or ceftaroline should be used instead. 1, 6, 7
Important Caveats
The most critical treatment for SSIs is surgical: opening the incision, evacuating infected material, and continuing dressing changes until healing by secondary intention. 1 Antibiotics are only necessary when there is >5 cm of erythema and induration, or when systemic signs are present (temperature >38.5°C, WBC >12,000 cells/µL, pulse >100 beats/minute). 1
For procedures involving the gastrointestinal tract, perineum, or female genital tract, the polymicrobial aerobic-anaerobic flora becomes more relevant, requiring broader coverage including gram-negative bacteria and anaerobes. 1, 2 However, the question describes a typical post-operative wound infection without specifying such anatomical locations, making S. aureus the most likely pathogen.