Erythema nodosum. Red, tender, subcutaneous nodules on the extensor aspects of the legs between knees and ankles are a common condition of many causes, some clear, some likely, and many obscure. The most important conditions that are heralded or attended by erythema nodosum are such infections as streptococcal upper-respiratory infections, ulcerative colitis, histoplasmosis, coccidioidomycosis, tuberculosis, syphilis, and leprosy. Another condition that is sometimes revealed by investigation of erythema nodosum is sarcoidosis. Drugs, including oral contraceptives, appear to be the cause of particular cases of erythema nodosum. In many cases, however, no clear cause can be found.
Erythema infectiosum (fifth disease). Erythema infectiosum is a mild childhood disease that is caused by human parvovirus B19. This condition develops after a mean incubation period of 14 days. There are few if any prodromal symptoms. The rash evolves in three clinical stages. The first stage is characterized by the abrupt appearance of a bright-red malar blush. The appearance is so startling that it has been given the suggestive description of “slapped cheeks”. During the second stage, the facial rash begins to fade, and a maculopapular, urticarial, or morbilliform exanthem develops on the extremities and trunk. Pruritus may be present.
Lichen simplex chronicus. Hyperpigmented, lichenified plaque with accentuated skin lined caused by repeated rubbing of the area. Lichen simplex chronicus (LSC) is a localized, well-circumscribed area of lichenification (thickened skin) resulting from repeated rubbing, itching, and scratching of the skin. It can occur on normal skin of individuals with atopic, seborrheic, contact dermatitis, or psoriasis.