What are two small, mold-like spots, one raised and the other flat, resembling a birthmark, on the back of a patient's thigh?

Differential Diagnosis of Two Small Mold-Like Spots on the Back of the Thigh

These lesions most likely represent benign cutaneous findings such as café-au-lait spots, melanocytic nevi, or capillary vascular malformations, which are common and typically require no intervention unless associated with other concerning features.

Primary Diagnostic Considerations

Benign Pigmented Lesions (Most Likely)

• Isolated café-au-lait spots and hypo/hypermelanotic macules or papules are classified as low-risk cutaneous markers that typically require only observation 1
• The flat lesion resembling a birthmark is consistent with a melanocytic nevus or café-au-lait macule, both of which are common benign findings 1
• The raised lesion could represent a slightly elevated nevus or small dermal nevus, which are normal variants in the general population 1

Vascular Malformations

• Capillary malformations (including nevus flammeus simplex or "salmon patches") are flat vascular lesions that can appear anywhere on the body and are present in up to 43% of the general population 1
• Nonmidline lesions on the thigh are not associated with underlying dysraphic malformations and are benign 1
• Infantile hemangiomas are raised, well-defined vascular lesions that develop in up to 5% of infants, though these typically appear in early infancy rather than being present at birth 1

Critical Features to Assess

Location and Midline Considerations

• The posterior thigh location is nonmidline and therefore not associated with spinal dysraphism or other congenital malformations 1
• Midline lumbosacral lesions would be concerning for underlying spinal cord abnormalities, but lateral thigh lesions lack this association 1

Morphologic Characteristics to Document

• Measure the exact size of both lesions (macules are defined as ≤0.5 cm and flat) 2
• Assess whether the raised lesion is truly elevated or simply appears darker/thicker 2
• Examine for any associated features such as hair growth (hypertrichosis), surrounding erythema, or satellite lesions 1

When to Pursue Further Evaluation

Red Flags Requiring Urgent Assessment

• Multiple café-au-lait spots (≥6 lesions >0.5 cm) would raise concern for neurofibromatosis type 1 and warrant genetic evaluation 1
• Rapid growth, color change, bleeding, or ulceration of either lesion requires dermatologic evaluation 1
• Associated systemic symptoms, pain, or signs of infection necessitate immediate assessment 1

Infectious Considerations (Less Likely)

• Cutaneous mold infections present as papules, nodules, or ulcers in immunocompromised patients, not as stable birthmark-like lesions 1
• True fungal skin infections (tinea) would show scaling borders and central clearing, not a mold-like appearance 3
• The term "mold-looking" likely refers to color/texture rather than actual fungal infection in an otherwise healthy patient 1

Recommended Management Approach

Initial Assessment

• Perform visual inspection documenting size, color, borders, and texture of both lesions 3
• Palpate to confirm one is truly raised and assess consistency 3
• Photograph lesions for future comparison if any concern exists 3

Observation Strategy

• For isolated, stable pigmented or vascular lesions on nonmidline locations, observation alone is appropriate 1
• Instruct caregivers to monitor for changes in size, color, or symptoms 1
• Routine follow-up at well-child visits is sufficient unless changes occur 1

When Dermatology Referral Is Indicated

• Uncertain diagnosis after initial assessment 3
• Presence of ≥6 café-au-lait spots or other features suggesting genetic syndromes 1
• Rapid growth or symptomatic lesions 1
• Parental anxiety requiring specialist reassurance 1

Common Pitfalls to Avoid

• Do not confuse benign pigmented lesions with infectious processes based solely on descriptive terms like "mold-looking" 1
• Avoid unnecessary imaging or biopsy for clearly benign, nonmidline cutaneous findings 1
• Do not overlook the need to count total café-au-lait spots on full body examination if multiple lesions are present 1
• Remember that truly concerning cutaneous markers (hypertrichosis, infantile hemangiomas, dermal sinus tracts) are associated with midline spinal locations, not lateral thigh placement 1