A 42-year-old male farmer from rural Maharashtra presents with a 3-day history of rapidly spreading erythema, edema, and warmth over his left leg following a minor cut while working in the field. On examination, he has a temperature of 38.5°C, tachycardia (HR 110/min), and poorly demarcated erythema with brawny edema extending from the ankle to mid-thigh. There is no fluctuance or abscess formation. Blood cultures are pending. Which is the most likely causative organism and the immediate management?

Explanation

## Clinical Diagnosis: Acute Cellulitis **Key Point:** The clinical presentation of poorly demarcated erythema, brawny edema, and systemic toxicity without localized fluctuance or abscess is classic for cellulitis, not abscess formation. **High-Yield:** Group A Streptococcus (GAS) is the most common cause of acute cellulitis in immunocompetent hosts. It spreads rapidly along tissue planes via hyaluronidase and other spreading factors, explaining the rapid progression over 3 days. ### Differential Diagnosis of Soft Tissue Infections | Feature | Cellulitis | Erysipelas | Abscess | Necrotizing Fasciitis | |---------|-----------|-----------|---------|---------------------| | **Borders** | Poorly demarcated | Well demarcated, raised | Fluctuant center | Ill-defined, rapid spread | | **Causative organism** | GAS, S. aureus | GAS (superficial) | S. aureus (often) | Polymicrobial or Clostridium | | **Depth** | Dermis + subcutaneous | Superficial dermis | Localized collection | Fascia + muscle | | **Systemic toxicity** | Mild to moderate | Moderate | Variable | Severe, rapid | | **Management** | IV antibiotics | IV antibiotics | I&D + antibiotics | Urgent surgical debridement | **Clinical Pearl:** The absence of fluctuance rules out abscess formation, which would require incision and drainage. The absence of severe systemic toxicity, rapid progression with skin necrosis, and crepitus rules out necrotizing fasciitis (which is a surgical emergency). ### Management Algorithm for Cellulitis ```mermaid flowchart TD A[Cellulitis diagnosis]:::outcome --> B{Systemic toxicity?}:::decision B -->|Mild, outpatient| C[Oral antibiotics: amoxicillin-clavulanate or cephalexin]:::action B -->|Moderate-severe or hospitalized| D[IV antibiotics]:::action D --> E{Risk factors for MRSA?}:::decision E -->|No| F[IV Penicillin G or 1st-gen Cephalosporin]:::action E -->|Yes: IV drug use, recent hospitalization| G[Vancomycin or Linezolid]:::action F --> H[Elevation, supportive care]:::action G --> H H --> I[Clinical improvement in 48-72 hours]:::outcome ``` **High-Yield:** For GAS cellulitis without MRSA risk factors, high-dose IV penicillin G (2–4 million units every 4–6 hours) or IV cephalosporin (ceftriaxone 1–2 g daily or cefazolin 1–2 g every 8 hours) is the standard of care [cite:Bailey & Love 27e Ch 6]. **Mnemonic: SSTI Causative Organisms** — **GASP** - **G**roup A Streptococcus (cellulitis, erysipelas) - **A**naerobes (necrotizing fasciitis, diabetic foot) - **S**taphylococcus aureus (abscess, MRSA in risk groups) - **P**seudomonas (water-related, immunocompromised)