This patient is manifesting what is known as autoeczematization, better known as the id reaction. An id reaction is characterized by the acute development of an erythematous, often extremely pruritic (but not invariably so), maculopapular, or vesicular rash. This eruption typically appears one to three weeks following a primary infection or simple dermatitis that is by definition distant from the site of the id reaction.
In this patient, the rash appeared on his back when the primary infection involved the feet. Id reactions are most commonly seen on the arms, legs, and trunk. They can be seen on the face, hands, and feet, although less commonly so. The lesions associated with id reactions are usually symmetrical (as in the patient pictured).
Patients with id reactions should be referred to a dermatologist or infectious disease specialist for follow-up. The most important aspect of therapy involves identifying and adequately treating the underlying infection or dermatitis. Acute care in the ED involves symptom relief with antihistamines for itching as well as the use of systemic and topical steroids.