Most Likely Diagnosis: Tinea Nigra
The most likely diagnosis is tinea nigra, a benign superficial fungal infection that should be confirmed with a KOH preparation or fungal culture, and if positive, treated with topical antifungals.
Clinical Reasoning
Why Tinea Nigra is Most Likely
Tinea nigra presents classically as an asymptomatic brown or black macule on the palms and soles, caused by Phaeoannelomyces werneckii 1. Your patient's presentation fits this perfectly:
- Location: Plantar surface (classic site)
- Appearance: Brown discoloration, nickel-sized
- Symptoms: Asymptomatic
- Duration: One week without change
- Age: Children are commonly affected 1
The key clinical pearl here is that tinea nigra is frequently confused with melanocytic lesions, which is the main diagnostic pitfall 1.
Diagnostic Approach
Immediate Next Steps
Perform a KOH preparation (potassium hydroxide microscopy) of skin scrapings from the lesion
- Look for pigmented septate hyphae
- This is quick, inexpensive, and can be done in-office
If KOH is negative or equivocal, send fungal culture
- P. werneckii will grow on standard fungal media
Dermoscopy (if available) can show characteristic pigmented network pattern without melanocytic features
What This is NOT
Melanoma is extremely unlikely in this scenario because:
- Melanoma on plantar surfaces in a 7-year-old is exceedingly rare
- The one-week timeline is too short for melanoma presentation
- Melanoma guidelines focus on adults and high-risk lesions 2, 3
Other less likely differentials include:
- Palmoplantar lichen planus (typically pruritic, has flat-topped plaques) 4
- Post-traumatic pigmentation or foreign body reaction 5
- Juvenile plantar dermatosis (presents differently with scaling and fissuring) 6
Management Algorithm
If Tinea Nigra is Confirmed:
Treatment: Topical antifungal therapy
- Topical azoles (ketoconazole 2% cream or clotrimazole) applied twice daily for 2-4 weeks
- Alternative: Whitfield's ointment (benzoic and salicylic acid)
- Prognosis is excellent with complete resolution expected
If Diagnosis Remains Uncertain After Negative Fungal Studies:
- Observe for 2-3 weeks with photographic documentation
- If the lesion persists, changes, or grows, refer to pediatric dermatology for possible biopsy
- Biopsy should be considered if there's any concern for atypical melanocytic proliferation, though this would be unusual in this presentation
Critical Pitfalls to Avoid
- Do NOT rush to biopsy without attempting fungal studies first—tinea nigra is benign and easily treated
- Do NOT dismiss as melanoma without proper evaluation—this causes unnecessary anxiety and procedures
- Do NOT assume all foot rashes in children are tinea pedis—the differential for pediatric foot dermatoses is broader 6
Red Flags That Would Change Management
Refer immediately to dermatology if:
- Lesion is raised, nodular, or ulcerated
- Rapid growth or color change occurs
- Multiple lesions appear
- Patient has family history of melanoma or dysplastic nevus syndrome
- Lesion has irregular borders, multiple colors, or asymmetry (ABCDE criteria)