Diagnosis: tinea corporis
A potassium hydroxide (KOH) preparation was performed. It showed septate hyphae and confirmed a diagnosis of tinea corporis. We ordered a periodic acid-Schiff (PAS) stain on the previous biopsy specimen, and it revealed septate hyphae in the stratum corneum that were not apparent on the original hematoxylin and eosin (H&E) stained sections.
Dermatophyte infections of the skin are known as tinea corporis or “ringworm.” Ringworm fungi belong to 3 genera, Microsporum, Trichophyton, and Epidermophyton. These infections occur at any age and are more common in warmer climates.1 The classic lesion is an annular scaly patch, sometimes with the concentric rings, as seen in our patient (FIGURE). The bruising was almost certainly caused by rubbing and scratching.
We suspected tinea coporis based on the physical characteristics of the rash and the fact that it did not respond promptly to topical steroids. Our suspicions were confirmed by the KOH prep. Inked KOH using chlorazol black E stain turns fungal hyphae black, which makes them easier to distinguish from keratinocyte cell walls.2
Differential of a nonspecific rash should include infections
The initial misdiagnosis was based on the histopathologic diagnosis of spongiotic dermatitis. Subacute spongiotic dermatitis is associated with intracellular and intercellular edema of the keratinocytes in the epidermis. This is a nonspecific finding seen in eczematous dermatitis and can be etiologically associated with a wide variety of clinical conditions, including allergic contact dermatitis, atopic dermatitis, nummular eczema, and, in this case, dermatophytosis.3
If a biopsy is performed for a nonspecific rash, the pathologist should be advised of the possibility of superficial fungal infection. Providing a history and the physical characteristics of the rash or a differential diagnosis will prompt the performance of a PAS stain. Otherwise, the diagnosis can be missed because fungal elements are often not visible on routine H&E stains.