What is the cause of this girl’s plantar lesion?

David L. Kaplan, MD—Series Editor

This 15-year-old girl presented for evaluation of new pigmented lesion on her right sole. It was asymptomatic and had not been there 2 to 3 months ago. There is no family history of melanoma. She was taking no medications, and she is a competitive equestrian.

What is the cause of this girl’s plantar lesion?

  1. Benign nevus
  2. Dysplastic nevus
  3. Malignant melanoma
  4. Tinea nigra
  5. Café au lait spot

Answer on next page

Answer: Tinea nigra

A potassium hydroxide (KOH) test confirmed the suspected diagnosis of tinea nigra. Tinea nigra is a rare superficial mycosis that usually is caused by Hortaea werneckii. It is an infrequent, asymptomatic infection that affects human palms and soles (palms more frequently). Due to its halophilic behavior, the natural habitat of this fungus comprises hypersaline environments, which is why it prefers the palm and sole.

Tinea nigra has a major clinical relevance because it can be mistaken for various types of nevi. In some reports, it even has been misidentified as melanoma. A pigmentary change in the skin results in a dark-colored macule due to the accumulation of a melanin-like substance in the fungus.

Café au lait spots typically are congenital. Nevi can appear suddenly but should not be KOH-positive.