The correct answer is erythema annulare centrifugum (EAC; choice “c”), one of the so-called “reactive erythemas,” which have several potential causes including streptococcal infections. The clinical picture and classic histologic pattern seen on biopsy effectively ruled out the other items in the differential diagnosis.
Dermatophytosis (choice “a”), a superficial fungal infection, would likely have demonstrated KOH positivity at its scaly leading edge.
Subacute cutaneous lupus (choice “b”) has many morphologic presentations, including annular scaly lesions with clearing centers, but definite signs (interface dermatitis, increased mucin or apoptosis) would have been seen on biopsy.
Cutaneous T-cell lymphoma (choice “d”) is one of a handful of malignancies that can present as a “rash,” but the biopsy effectively ruled it out.
First described by Darier in 1916, EAC exhibits a pathognomic feature called “trailing scale.” With dermatophytosis, for example, the bulk of the scaling is found at the leading edge of the lesion. By contrast, with EAC, the scaling lags behind the leading erythematous border and is thus said to “trail” the edge. Since EAC so closely resembles dermatophytosis, this feature serves to distinguish the two. EAC lesions can be large, appear in multiples, and spare only palms and soles.
EAC is most often idiopathic. However, there are many reported triggers, including drugs (eg, gold, antimalarials, penicillin, and aspirin), infections (such as strep and dermatophytosis), and malignancies.
A brisk inflammatory tinea can set it off, complicating the clinical picture by presenting with two different types of annular lesions. Treating the fungal infection or other trigger effectively cures the EAC. In such cases, a careful history will show which rash came first.
When the cause is not immediately obvious, potential triggers that should be investigated include: occult malignancy, tuberculosis, sarcoidosis, and pregnancy. Examination of skin distant from the obviously affected areas is often necessary, with special attention to the feet, groin, and scalp. Laboratory studies should include a complete blood count, liver function tests, rapid plasma reagin test, urinalysis, and chest x-ray for completeness.
EAC is treated, as mentioned earlier, by finding and eliminating the underlying cause. That often proves impossible, so steroid therapy (oral or topical) is often employed with some effect (though it will not prevent recurrences of the condition). Empiric use of oral antibiotics has produced mixed results. The condition can persist for years but is seldom significantly symptomatic.