Diagnosis: Cellulitis
The diagnosis is C. Cellulitis. This pediatric patient's presentation of erythematous swelling with tender, indistinct margins on the leg, accompanied by fever, lymphangitis, and lymphadenitis, is pathognomonic for cellulitis 1.
Clinical Reasoning
Cellulitis is defined as an acute bacterial infection of the dermis and subcutaneous tissue that presents with:
- Erythema, warmth, and tenderness 1
- Indistinct (poorly demarcated) borders - a key distinguishing feature 1, 2
- Edema and induration 1
- Lymphangitis (inflammation of lymphatic vessels) 1
- Regional lymphadenitis (lymph node inflammation) 1
- Systemic symptoms including fever 1
The presence of lymphangitis and lymphadenitis strongly supports cellulitis, as these features indicate spreading infection through the lymphatic system 1.
Why Not the Other Options
Folliculitis (Option A) is excluded because:
- Folliculitis presents as small, localized pustules centered around hair follicles 1
- It does not cause diffuse erythematous swelling with indistinct margins 1
- Lymphangitis and lymphadenitis are not features of folliculitis 1
Impetigo (Option B) is excluded because:
- Impetigo presents as discrete purulent lesions or honey-crusted erosions confined to the superficial epidermis 1
- It does not cause deep tissue swelling or induration 1
- Lymphangitis is not a typical feature 1
Necrotizing fasciitis (Option D) is excluded because:
- While necrotizing fasciitis can present with erythema and systemic toxicity, this patient lacks the hallmark severe features 1
- Necrotizing fasciitis typically presents with severe pain out of proportion to physical findings, rapidly progressive skin changes (including bullae, crepitus, skin necrosis), and signs of systemic toxicity with hypotension and altered mental status 1
- The clinical description suggests a more superficial process consistent with cellulitis 1
Management Implications
This pediatric patient with cellulitis requires:
- Empiric antibiotic therapy targeting Gram-positive bacteria, particularly streptococci and Staphylococcus aureus 1, 3
- Beta-lactam antibiotics are first-line for typical cellulitis without purulent features 1, 2
- MRSA coverage should only be added if there are purulent features, penetrating trauma, or failure to respond to beta-lactam therapy 2
- The presence of systemic symptoms (fever) and lymphangitis indicates this is not a simple superficial infection and warrants systemic antibiotic therapy 1
Critical pitfall to avoid: Do not confuse the indistinct margins of cellulitis with the well-demarcated borders of erysipelas, which affects more superficial layers and has sharp, raised borders 1.