Figure. Solitary reddish-brown, rough-surfaced papule on the leg

A 64-year-old woman has had a several-month history of a gradually enlarging lesion on her leg. The lesion began as a brown spot, but it has turned red and has started to itch. She traumatizes the lesion occasionally while shaving.

On examination, there is a sharply demarcated, reddish-brown papule, with a slightly roughened surface, indicating overlying scale (figure). The lesion has no substance other than what one could feel at the surface.

Differential diagnosis Consider conditions that produce a solitary, rough-surfaced papule on the leg.

A wart can produce a solitary rough-surfaced papule, however, warts are not usually red in color.

Squamous cell carcinoma may present as a solitary, scaly papule on the leg. The lack of substance in the underlying tissue makes this diagnosis much less likely.

Seborrheic keratosis commonly occurs on the leg as a solitary, sharply demarcated papule, very similar to what is seen in this case. However, the lesions are typically brown, blue, flesh-colored, or black, and are not red in color.

Psoriasis can appear as a solitary, scaly, reddish-brown papule or plaque. If this patient had many other similar lesions, this diagnosis would be more likely.

Benign lichenoid keratosis is the correct diagnosis. This lesion typically arises in a pre-existent pigmented lesion, either a lentigo or a seborrheic keratosis. For unknown reasons, an inflammatory process occurs and produces the erythema that is present almost universally. This inflammatory response may subside and the lesion may revert back to its previous morphology.

The lesion was treated with destruction by electrodesiccation and curettage.

Diagnostic pearl Consider benign lichenoid keratosis if a preexistent lentigo or seborrheic keratosis suddenly becomes symptomatic and turns red.

Dr. Levine is professor of medicine (dermatology), University of Arizona Health Sciences Center, Tucson.