Question:

A 40-year old lady presented with an asymptomatic brown nodule on her left lower leg for one year, which she noticed to have developed following an insect bite reaction at the same site. On palpation the nodule was firm and situated within the dermis.

Answer: D. Dermatofibroma

Dermatofibromata are usually asymptomatic and found on the lower legs of young adults, often women. They may be solitary or multiple, grow slowly and remain stationary for years. They represent a reaction pattern to an insect bite or other trauma, but such a history can not always be obtained. Palpation reveals a 5-10mm firm dermal nodule exhibiting the "iceberg" effect, as it feels much larger within the dermis than it looks on the skin surface. The lesion is firmly attached to overlying skin and its surface is bound down, with puckering to the overlying hyperkeratotic pigmented epidermis. The "dimple sign" is demonstrated when pinching the lesion, as lateral compression of the lesion between thumb and index finger produces a depression or dimple. Histology consists of a dermal nodule with interwoven mesh of histiocytes and collagen, with overlying epidermal hyperplasia. A pigmented dermatofibroma may be confused with a melanocytic naevus or malignant melanoma. Excision of symptomatic or diagnostically doubtful lesions is recommended.

Prurigo nodularis is often itchy and excoriated. Wart is a hyperkeratotic epidermal lesion without an underlying dermal nodule on palpation. Haemangioma is a vascular lesion that is red rather than brown in colour, and blanches on pressure.