What is the diagnosis and treatment for a 7-year-old female with an erythematous patch excoriated on her ankle, with no other medical problems?

Erythematous Excoriated Patch on Ankle in 7-Year-Old

The most likely diagnosis is atopic dermatitis (eczema) with secondary excoriation from scratching, and treatment should consist of liberal emollient application, a mild-to-moderate potency topical corticosteroid, and evaluation for secondary bacterial infection if crusting or weeping is present. 1

Diagnostic Approach

Primary Differential Diagnosis

The ankle location with excoriation in a 7-year-old suggests several key possibilities:

• Atopic dermatitis is the most common cause of chronic pruritic dermatitis in children, affecting 5-15% of schoolchildren, and characteristically presents with itchy skin conditions that lead to scratching and excoriation 1
• Impetigo presents as erythematous papules that evolve into vesicles/pustules, then rupture forming honey-colored crusts on an erythematous base, typically on exposed areas including extremities 2
• Contact dermatitis from irritants (soaps, detergents) or allergens can cause erythematous, vesicular lesions with excoriation from scratching 1
• Tinea corporis presents with well-demarcated, circular, scaly patches with raised leading edges, though excoriation may obscure typical features 2, 3

Critical Features to Assess

History elements that distinguish diagnoses:

• Duration and pattern (acute vs. chronic/relapsing) - chronic favors atopic dermatitis 1
• Personal or family history of atopy (asthma, hay fever, eczema) - strongly suggests atopic dermatitis 1
• Pruritus severity - intense itching with scratching/rubbing is the hallmark of atopic dermatitis 1
• Recent tick exposure or outdoor activities in endemic areas - erythema migrans appears 7-14 days post-tick bite and should be ≥5 cm for secure diagnosis 1

Physical examination findings:

• Presence of honey-colored crusting suggests bacterial superinfection (impetigo) requiring antimicrobial therapy 2
• Weeping or crusting indicates possible Staphylococcus aureus infection complicating eczema 1
• Scaly borders with central clearing favor tinea corporis - confirm with KOH preparation 2, 3
• Expanding erythematous patch without significant pruritus suggests erythema migrans, though vesicles occur in only ~5% of cases 1

Treatment Algorithm

For Atopic Dermatitis (Most Likely)

First-line management:

• Liberal emollient application is essential - prescribe adequate quantities and educate on frequent use throughout the day 1
• Topical corticosteroids appropriate for age, site, and disease extent - use mild-to-moderate potency for ankle in a child 1
• Keep nails short to minimize excoriation damage 1

If secondary bacterial infection suspected (crusting/weeping):

• Culture the lesion to establish causative organism 2
• Topical mupirocin or retapamulin for localized infection 2
• Oral antibiotics (dicloxacillin, cephalexin) if extensive - most S. aureus from impetigo are methicillin-susceptible 2
• For penicillin allergy or MRSA: doxycycline (appropriate for age 7), clindamycin, or trimethoprim-sulfamethoxazole 2

For Alternative Diagnoses

If tinea corporis suspected:

• Confirm with KOH preparation of skin scrapings 2, 3
• Topical antifungal agents (azoles or allylamines) for treatment 4

If erythema migrans suspected (Lyme disease):

• Diagnosis is clinical based on visual inspection in endemic areas - serologic testing is too insensitive in early disease 1
• Oral antibiotics: amoxicillin (preferred in children <8 years), doxycycline (age ≥8 years), or cefuroxime for 10-21 days 4

Critical Pitfalls to Avoid

• Do not assume simple irritation - deterioration in previously stable eczema may indicate secondary bacterial infection or development of contact dermatitis 1
• Do not delay treatment for laboratory confirmation in clinically obvious atopic dermatitis - diagnosis is clinical 1
• Do not overlook Lyme disease in endemic areas - erythema migrans can occur on lower extremities and may be partially purpuric 1
• Do not use topical corticosteroids alone if bacterial infection is present - this can worsen the infection 1
• Ensure adequate education on proper application technique and quantity of topical preparations - inadequate treatment is a common cause of apparent treatment failure 1