In mid-December, a woman with insulin-dependent diabetes who had been on prednisone fell and received an abrasion on the dorsal side of her right hand. She was placed on penicillin. By the end of January, the ulcer had not healed, and she was referred to a plastic surgeon. On January 30, a swab of the wound was cultured at 35°C on blood agar. On the same day, a smear was made for Gram staining. The Gram stain showed large (10 um) cells. Brownish, waxy colonies grew on the blood agar. Slide cultures set up on February 1 and incubated at 25°C showed septate hyphae and single conidia. The most likely cause of the infection is a
A) Gram-negative bacterium.
B) Dimorphic fungus.
C) Parasitic alga.
D) Yeast.
E) Pleomorphic protozoan.

Dimorphic fungi are fungi that change forms depending on the ambient environmental conditions. Thermally dimorphic fungi are a subset of the dimorphic fungi that specifically change forms as a result of the prevailing temperature. They can exist either as unicellular yeast or as multicellular mold.

In yeast form, dimorphic fungi are visible as single cells when stained and observed under the microscope. They posses filamentous septate or aseptate hypha that is characteristic of multicellular fungi when they take up the mold form.

From the observations in the case of the diabetic woman in question, the cultured swab at [tex]35^{o}[/tex] C indicated the presence of unicellular yeast. When the procedure was repeated at [tex]25^{o}[/tex] C, it indicated the presence of multicellular mold.

The observations indicated that the likely cause of the infection is a Dimorphic fungus.