Why does this woman have an asymptomatic rash on her left lower back?

A 54-year-old female presented for evaluation of an asymptomatic rash on her left lower back of 3 months duration. She is otherwise healthy. She owns 2 dogs but has had them for several years and they have no skin issues. Her husband is not affected. 

What is responsible for her rash?

A. Asteatotic eczema

B. Ringworm (dermatophyte infection)

C. Candidiasis

D. Erythema annulare centrifugum

E. Contact dermatitis

Answer: Erythema annulare centrifugum

A skin biopsy confirmed the clinical impression of erythema annulare centrifugum. The benign reactive process mimics ringworm with eccentric rings that expand slowly, break up, resolve, and then reappear. It is probably due to a hypersensitivity reaction to a variety of agents—including drugs, arthropod bites, infections (bacterial, mycobacterial, viral, fungal, filarial), ingestion (blue cheese), and malignancy. An extensive work-up is not warranted. Asteatoses is possible, but is usually itchy and the nummular patches are distinct and do not coalesce as seen here. Dermatophyte infection needs to be ruled out by potassium hydroxide, culture, or biopsy. Candidiasis is unlikely in a healthy patient. Contact dermatitis would be expected to be pruritic. ν