How would you diagnose this itchy rash?

A 35-year-old man presented for evaluation of a slowly worsening itchy rash on both feet and ankles over 2 weeks this past summer. He denies any new exposure history and is otherwise healthy.

How would you diagnose this rash?

A. Dermatophyte infection
B. Contact dermatitis to shoes
C. Insect bite reaction
D. Erythema multiforme
E. Impetigo 

Answer on next page

Answer: Dermatophyte infection

This patient had a bullous dermatophyte infection (typically Trichophyton mentagrophytes) on his feet manifested by scaling, erythema, pruritus, and occasional vesicles. Potassium hydroxide (KOH) test of the scale is usually positive, while the blisters are often sterile and are a reaction to the dermatophyte. 

In addition, the patient had developed a hypersensitivity reaction to the dermatophyte infection manifested by the discrete erythematous papules coming up onto the ankle and lower leg. He responded to a combination of antifungal therapy and a short prednisone taper with complete resolution of his infection and secondary reaction.

Differential diagnosis. Contact dermatitis would typically be confined to the dorsum of the foot and not be discrete papules as seen here or involve the toes’ webs. Insect bites can produce a bullous reaction, but only at the bite site, and they would not result in this clinical pattern. Erythema multiforme is typically on the palms and soles with target-like lesions that are both itchy and tender. Impetigo would be similar but be ruled out by the positive KOH test; furthermore, lesions on the ankles are not characteristic of impetigo.