A 32-year-old woman presents with acute onset of fever, chills, and a painful swollen lesion on her left forearm. On examination, there is a localized area of erythema, warmth, and fluctuance. Gram stain of the aspirated pus shows gram-positive cocci in clusters. Culture confirms Staphylococcus aureus. Which is the most common site of S. aureus skin and soft tissue infection in community settings?

## Most Common Site of S. aureus Skin and Soft Tissue Infection ### Epidemiology of Community-Acquired S. aureus Infections **Key Point:** The extremities (arms and legs) are the most common sites for community-acquired S. aureus skin and soft tissue infections, particularly in areas prone to minor trauma, insect bites, or abrasions. ### Anatomical Distribution The following table summarizes the relative frequency of S. aureus skin infections by anatomical site: | Site | Frequency | Clinical Presentation | Risk Factors | | --- | --- | --- | --- | | **Extremities (arms, legs)** | Most common (40–50%) | Abscesses, cellulitis, impetigo | Minor trauma, cuts, abrasions | | Face and neck | Common (20–25%) | Folliculitis, boils, impetigo | Shaving, poor hygiene | | Buttocks and perineum | Less common (10–15%) | Furuncles, carbuncles | Friction, moisture, poor hygiene | | Trunk and back | Least common (5–10%) | Folliculitis, abscesses | Friction from clothing | ### Clinical Pearl **Clinical Pearl:** Community-acquired methicillin-resistant S. aureus (CA-MRSA) classically presents as a recurrent boil or abscess on the extremities in otherwise healthy individuals, often misdiagnosed as a "spider bite" because of the localized erythema and central drainage. ### Pathophysiology The extremities are favored sites because: 1. **Increased trauma exposure** — hands and legs encounter more minor cuts and abrasions in daily activities. 2. **Colonization of hair follicles** — S. aureus colonizes the pilosebaceous units, which are abundant on limbs. 3. **Poor wound care** — minor injuries on extremities are often left uncovered or contaminated. ### High-Yield Fact **High-Yield:** In hospitalized patients, S. aureus skin infections more commonly affect the buttocks and perineum due to prolonged sitting, moisture, and friction from bedding. However, in community settings, extremities predominate. ### Distinction from Other Pathogens - **Streptococcus pyogenes** — also causes cellulitis but typically on the face and lower legs; spreads rapidly with less localization. - **Clostridium perfringens** — causes necrotizing fasciitis, usually on the trunk or perineum after trauma or surgery. [cite:Harrison 21e Ch 297]