What could be the cause of this new rash?

A 40-year-old male presented with a 2-week history of rhus contact dermatitis (top). Initial treatment with a methylprednisolone dose pak only partially cleared the eruption. This subsequent visit revealed a new rash (bottom) in a different location from the original without any new exposure history. 

What could be the cause of this new rash?

A. Delayed contact dermatitis to rhus exposure due to the corticosteroids
B. Drug reaction to the methylprednisolone
C. Dermatophyte exacerbation due to the immunosuppression of the steroids
D. Erythema annulare centrifugum secondary to rhus contact dermatitis or the steroids
E. Repeat exposure to rhus contact dermatitis from unsuspected source (clothes, pet, etc.)

(Answer on next page)

Answer: Erythema Annulare Centrifugum (EAC)

Though there were a number of differential diagnoses, it was determined that this secondary eruption represented EAC from either the contactant (most likely) or the steroid dose pack (less likely). EAC is poorly understood but probably represents a hypersensitivity reaction and has been associated with a variety of etiologies. This patient was undertreated with his corticosteroids as many times a steroid dose pack provides inadequate dosing and insufficient duration to eradicate a contact dermatitis. Repeat steroid taper at a higher dose and longer duration eliminated both problems for this patient.

Answers A and E are not unreasonable though the appearance of trailing scale suggests the clinical diagnosis of EAC rather than a contact dermatitis. Dermatophyte would typically not spread that quickly and a 6-day prednisone taper would not be considered immunosuppressive in a healthy host. A drug reaction would typically be more generalized than seen here.