<unk>  usually arises due to an underlying epidermal lesion, the most common being verruca vulgaris (wart), actinic keratosis (a potentially pre-malignant lesion of dysplastic keratinocytes), or squamous cell carcinoma (a form of skin cancer). These can look essentially identical clinically. The only reliable way to diagnose which type of lesion is present under the<unk>  is to biopsy the lesion and have it microscopically examined by a pathologist (or dermatologist).
Cutaneous horn
DermAtlas
Dermatome 
Dermatome
Dermal patch
Pinard horn
Neuromatrix
Ecamsule