Enlarging pigmented lesion on the foot


Fig 1. Tan patch
Figure 1. Mottled, tan-coloured patch on the sole of the foot.
Fig 2. Pigmented spores
Figure 2. Skin biopsy demonstrating pigmented spores and hyphae in the keratin layer.

Over a nine-month period, a 42-year-old man developed an enlarging, mottled, tan-coloured lesion on the sole of his left foot (Figure 1). A skin biopsy showed multiple pigmented spores and hyphal elements within the keratin layer (Figure 2).

Differential diagnosis

The following conditions should be considered in the differential diagnosis of a pigmented lesion on the sole of the foot.
  • Café au lait macules may have an identical colour to this man’s lesion, but they appear in childhood, have an even colour rather than a mottled pattern, and are usually not on the sole. Biopsy shows epidermal hyperpigmentation.

  • Pigmented epidermal naevi also usually have their onset in childhood and have a more whorled or linear appearance. Biopsy often shows verrucose epidermal hyperplasia.

  • Chemicals such as self-tanning lotions may produce discolouration of the skin, but usually this is short lived and lacks the history of progressive growth.

  • Malignant melanoma of the sole is an important differential diagnosis. Malignant melanoma usually has a more variegated colour spectrum and dermoscopy shows a broad ridge pattern of pigmentation. Skin biopsy will help confirm the diagnosis prior to surgery.

  • Tinea nigra is the correct diagnosis in this case. It is caused by the presence of a superficial mould called Hortaea (Exophiala) werneckii. The mould is able to synthesise melanin and masquerade as a melanoma. The infection is usually localised to either a palm or sole. The diagnosis can be confirmed by skin scraping and culture or by skin biopsy. The mould will respond to topical imidazole creams, such as 2% miconazole or 2% ketoconazole, and also to keratolytic agents such as urea or Whitfield ointment (benzoic acid, salicylic acid).


Tinea nigra may masquerade as a malignant melanoma, but the limited colour range and mottled pattern are clues. Skin scraping or a biopsy should be obtained to avoid unnecessary surgery.

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