A 32-year-old man from rural Rajasthan presents with a 3-week history of progressive swelling and erythema of the left leg following a minor cut on the shin. On examination, he has a sharply demarcated, brawny, non-pitting edema with a raised, advancing border. The area is warm and tender. Culture of fluid from the lesion grows β-hemolytic streptococci sensitive to penicillin. He also has fever (38.2°C) and regional lymphadenopathy. What is the most likely diagnosis?

Explanation

## Erysipelas: Classic Streptococcal Superficial Skin Infection ### Clinical Presentation Analysis The patient presents with: - **Sharply demarcated, raised, advancing border** — the hallmark of erysipelas - **Brawny, non-pitting edema** — characteristic of erysipelas (lymphatic involvement) - **Warm, tender, erythematous plaque** following a minor skin breach - **Fever (38.2°C) and regional lymphadenopathy** - **β-hemolytic streptococci (GAS)** sensitive to penicillin — the causative organism in >90% of erysipelas cases ### Erysipelas: Key Features **Key Point:** Erysipelas is an acute, superficial bacterial skin infection involving the dermis and superficial lymphatics, caused almost exclusively by *Streptococcus pyogenes* (Group A Streptococcus). The **sharply demarcated, raised, palpable border** is its pathognomonic feature, distinguishing it from deeper cellulitis. *(Harrison's Principles of Internal Medicine, 21e, Ch. 124; Robbins & Cotran Pathologic Basis of Disease, 10e, Ch. 25)* | Feature | **Erysipelas** | Cellulitis | |---|---|---| | **Depth** | Dermis + superficial lymphatics | Dermis + subcutaneous tissue | | **Border** | **Sharply demarcated, raised, palpable** | Ill-defined, blends into surrounding skin | | **Edema** | Brawny, non-pitting (lymphatic obstruction) | Variable, less pronounced | | **Surface** | "Peau d'orange" (orange-peel) appearance | Smooth | | **Causative organism** | GAS (>90%) | GAS, *S. aureus* (most common) | | **Antibiotic of choice** | **Penicillin V / Amoxicillin** | Cephalosporin / Cloxacillin | | **Recurrence** | Higher (chronic lymphedema risk) | Lower | ### Why This Is Erysipelas, Not Cellulitis **High-Yield:** The **sharply demarcated, raised, advancing border** is the single most important distinguishing feature of erysipelas. The original explanation incorrectly stated that brawny non-pitting edema and raised border favor cellulitis — in fact, the **raised, sharply demarcated border** is the classic hallmark of **erysipelas**, not cellulitis. Cellulitis has an **ill-defined border** that blends into surrounding normal skin. **Clinical Pearl:** The distinction matters for treatment: - **Erysipelas** → Penicillin V or amoxicillin (GAS-specific; excellent response — consistent with the culture showing penicillin-sensitive β-hemolytic streptococci) - **Cellulitis** → Broader coverage (cephalosporin or cloxacillin) to cover *S. aureus* as well ### Pathophysiology ``` Minor skin breach (cut/abrasion) ↓ GAS inoculation → invasion of dermis + superficial lymphatics ↓ Erysipelas: sharply demarcated, raised border, brawny non-pitting edema ↓ Regional lymphadenopathy + systemic fever ↓ Treatment: Penicillin V (GAS-specific, penicillin-sensitive) ``` ### Why NOT the Other Options - **A) Impetigo:** Superficial, crusted, vesiculopustular lesions — no deep edema, no raised border, no systemic features. Typically in children. - **B) Cellulitis:** Deeper infection (dermis + subcutaneous tissue) with **ill-defined borders** that blend into surrounding skin. The sharply demarcated, raised border in this case argues against cellulitis. - **C) Necrotizing fasciitis:** Rapidly progressive, involves deep fascia; presents with severe pain out of proportion, skin necrosis, crepitus, and systemic toxicity (septic shock). Culture would show mixed organisms or GAS with tissue destruction. This patient lacks these features. ### Management - **First-line:** Oral Penicillin V (500 mg QID × 10–14 days) or Amoxicillin - **Severe/systemic:** IV Benzylpenicillin - **Penicillin allergy:** Erythromycin or Clindamycin - **Supportive:** Limb elevation, analgesia, treat predisposing factors (tinea pedis, lymphedema)