What is the next best step for a 49-year-old male patient with a rash on the left shin that has not responded to clotrimazole treatment?

Skin Biopsy is Indicated for Persistent Rash on Left Shin

A skin biopsy is the next best step for this 49-year-old male patient with a persistent, enlarging rash on the left shin that has not responded to antifungal treatment. 1

Clinical Assessment and Rationale

The clinical presentation raises several concerning features:

• Rash persisting for over a year
• Progressive increase in size
• Failure to respond to antifungal treatment (clotrimazole)
• Location on the shin (a common site for various non-infectious dermatoses)

These features suggest this is not a simple fungal infection and warrants histopathological evaluation to establish a definitive diagnosis.

Diagnostic Algorithm

1. Failure of empiric antifungal therapy: The lack of response to clotrimazole after 30 days strongly suggests a non-fungal etiology 2, 3

2. Duration and progression: A rash persisting for over a year with continued enlargement despite treatment is concerning for:

   • Inflammatory dermatoses
   • Neoplastic processes
   • Autoimmune conditions
   • Pyoderma gangrenosum (especially on the shin) 4

3. Location on shin: The pretibial area is a common site for:

   • Pyoderma gangrenosum (especially with underlying inflammatory bowel disease)
   • Necrobiosis lipoidica
   • Cutaneous lymphoma
   • Chronic venous stasis dermatitis

Biopsy Technique Considerations

The European Society for Medical Oncology (ESMO) and American Society of Clinical Oncology (ASCO) guidelines recommend:

• Pertinent history and physical examination including examination of the oral mucosa, assessment for blister formation, and assessment of body surface area involved 1
• Review of medications to rule out drug-induced causes 1
• Skin biopsy for persistent, undiagnosed rashes 1

For this patient, a punch biopsy (4-6mm) would be most appropriate, as it:

• Provides adequate tissue for histopathological examination
• Allows assessment of the full thickness of the dermis
• Has minimal scarring
• Can be performed in an outpatient setting

Common Pitfalls to Avoid

1. Prolonged empiric therapy: Continuing antifungal treatment despite lack of response delays proper diagnosis and treatment 1

2. Misdiagnosis as cellulitis: Non-infectious inflammatory conditions like pyoderma gangrenosum are frequently misdiagnosed as cellulitis, leading to inappropriate antibiotic treatment 4

3. Inadequate sampling: Superficial shave biopsies may miss deeper pathology; punch or incisional biopsies are preferred for persistent, undiagnosed rashes 1

4. Failure to consider systemic associations: Some skin conditions (like pyoderma gangrenosum) can be associated with underlying systemic diseases that require evaluation 4

Post-Biopsy Management

While awaiting biopsy results:

• Consider topical high-potency corticosteroids if inflammation is present 1, 5
• Avoid further irritation to the area
• Document the size and appearance of the rash (photography if available)
• Plan for follow-up within 1-2 weeks to review biopsy results

The biopsy results will guide definitive treatment, which may include topical therapies, systemic medications, or referral to specialists depending on the diagnosis.