Nevus depigmentosus (achromicus). These are localized areas of hypopigmentation that are usually present at birth. The lesions may be irregular in size and shape and occasionally follow a linear or segmental pattern. Electron microscopic study of these areas suggests that melanosomes are not being transferred from melanocytes into surrounding keratinocytes. There are no associated abnormalities.
Phototoxic dermatitides. In phototoxic reactivity, no immunologic mechanism is involved, and the patient reacts as anyone would to a primary irritant. Phototoxic drugs and chemicals include some dyes, coal tar derivatives, and psoralens. Drugs that may cause a phototoxic reaction include the sulfonamides, tetracyclines, and thiazides.
Hand-foot-mouth disease. This common and benign viral disease of childhood is usually caused by the A16 strain of coxsackievirus, although other strains of the same virus have been implicated. It most often occurs in late summer and early fall. The prodrome consists of low-grade fever and malaise. Shortly thereafter, vesicular lesions arise on the soft palate, tongue, buccal mucosa, and uvula. The lips are usually spared. Occasionally, these lesions may be painful and cause some difficulty in eating. The cutaneous lesions develop 1 or 2 days after those in the mouth. They consist of asymptomatic round or oval vesiculopustules that evolve into superficial erosions. The edges of the palms and soles are a favored location.
Acanthosis nigricans. This is a descriptive term for a velvety or verrucous brown-to-black area of hyperkeratosis. The axilla and posterior are the most common locations, but lesions are also seen on the anterior neck and in the groin. Less commonly, there is involvement in the antecubital and popliteal fossae, on the knuckles, and in other, nonflexural areas. Onset may occur during childhood or adult life. The histologic pattern is that of hyperkeratosis and papillomatosis; the brownish discoloration seems to be caused by these surface changes rather than by any local increase in the amount of melanin. Illustrated here are lesions of acanthosis nigricans on the anterior neck and in the axilla.
Urticaria. Wheals with white-to-light-pink color centrally and peripheral erythema in a close-up view. These are the classic lesions of urticaria. It is characteristic that they are transient and highly pruritic.