What could be the cause of a localized skin lesion on the forearm that resembles a burn, started as a pimple, has spread, and is weeping?

Differential Diagnosis: Weeping Forearm Lesion

This presentation most likely represents an infected skin lesion requiring urgent evaluation with skin biopsy and culture to rule out serious infections including invasive fungal infection, bacterial cellulitis with abscess formation, or atypical mycobacterial infection.

Key Clinical Features to Assess Immediately

The progression from a "pimple-like" lesion to a spreading, weeping, burn-like appearance on the forearm raises several concerning possibilities:

Primary Differential Considerations

Bacterial infection (most common):

• Infected epidermoid/sebaceous cyst - Can present initially as a small papule that becomes inflamed, ruptures, and develops surrounding cellulitis with weeping discharge 1
• Cellulitis/erysipelas - Spreading erythematous infection that can appear on extremities 2
• Look for: warmth, tenderness, expanding erythema, purulent drainage, fever 1

Invasive fungal infection (serious, requires urgent diagnosis):

• Cutaneous aspergillosis - First appears as erythematous papules, becomes pustular, then develops central ulceration with elevated border covered by black eschar; occurs on extremities 3
• Invasive dermatophyte infection (Trichophyton rubrum) - Can present as symmetric lesions on forearms with atypical morphology, particularly in immunocompromised patients 4
• Look for: black eschar formation, lack of tenderness initially, immunocompromised state 3

Atypical presentations:

• Eczema vaccinatum-like process - Papular, vesicular, or pustular rash with weeping, particularly if patient has history of atopic dermatitis 3
• Burn wound secondary infection - If there was any preceding thermal injury 5

Immediate Diagnostic Steps

Obtain skin biopsy with fungal culture immediately - This is critical as the appearance is not pathognomonic and definitive diagnosis requires histopathology 3

• Use scalpel (not laser/electrocautery) to preserve tissue architecture 1
• Send for: bacterial culture, fungal culture, and histopathology 3, 1
• Document: age, sex, exact anatomic location 1

Assess for systemic involvement:

• Fever, tachycardia, tachypnea, hypotension indicate need for hospitalization 1
• Extensive surrounding cellulitis suggests spreading infection requiring admission 1
• Immunocompromised status (transplant, chemotherapy, chronic granulomatous disease) dramatically increases risk of invasive fungal infection 3

Management Algorithm

If Patient Appears Systemically Well:

• Initiate topical mupirocin 2% ointment for empiric coverage of Staphylococcus aureus and streptococci 6
• Apply chlorhexidine gluconate solution to reduce bacterial colonization 6
• Keep area clean and dry between applications 6
• Close outpatient follow-up in 24-48 hours to reassess 1

If Patient Has Systemic Signs or Immunocompromise:

• Immediate hospitalization for suspected invasive infection 1
• Start systemic antifungal therapy empirically if invasive fungal infection suspected (particularly if black eschar develops or patient is immunocompromised) 3
• Surgical debridement may be necessary if infection progresses 3

Critical Red Flags Requiring Urgent Action

• Black eschar formation - Highly suggestive of Aspergillus cutaneous infection requiring systemic antifungals 3
• Painless progressive necrosis - Consider invasive fungal infection 3
• Immunocompromised host - Dramatically increases risk of life-threatening invasive infections 3
• Rapid progression despite initial treatment - Requires biopsy and culture before continuing empiric therapy 3

Common Pitfalls to Avoid

Do not delay biopsy - The appearance of cutaneous Aspergillus is characteristic but not pathognomonic; other infections can appear similar, and definitive diagnosis requires tissue 3

Do not assume simple bacterial infection - In immunocompromised patients, cutaneous fungal infections result from hematogenous seeding and indicate disseminated disease requiring aggressive systemic therapy 3

Do not use destructive sampling methods - Laser or electrocautery compromise histological diagnosis; always use scalpel excision 1

Monitor for progression beyond 15 days - Lesions that fail to heal suggest underlying immunodeficiency or atypical pathogen 3