**Question:** Which of the following is not a dimorphic fungus?

A. Aspergillus fumigatus
B. Histoplasma capsulatum
C. Cryptococcus neoformans
D. Paracoccidioides brasiliensis

**Core Concept:** Dimorphic fungi are a group of pathogenic fungi that can exist in two different morphological forms: yeast and mold. Some examples include Histoplasma capsulatum, Aspergillus fumigatus, Cryptococcus neoformans, and Paracoccidioides brasiliensis.

**Why the Correct Answer is Right:** Paracoccidioides brasiliensis is a dimorphic fungus, capable of switching between yeast and mold forms. This yeast-mold dimorphism is a defining characteristic of dimorphic fungi.

**Why Each Wrong Option is Incorrect:**

A. Aspergillus fumigatus is a dimorphic fungus, exhibiting yeast and mold forms.
B. Histoplasma capsulatum is another example of a dimorphic fungus, transitioning between yeast and mold forms.
C. Cryptococcus neoformans is another dimorphic fungus, changing between yeast and mold forms.

**Clinical Pearl:** Dimorphic fungi are often found in soil, especially in regions with warm, humid climates. When inhaled into the lungs, they can cause severe systemic infections in humans.

**Correct Answer:** Paracoccidioides brasiliensis is not a dimorphic fungus, which is why it is the correct choice in this multiple-choice question. However, it is essential for medical students to be familiar with the yeast-mold dimorphism exhibited by dimorphic fungi, as this knowledge is crucial for diagnosing and treating infections caused by these organisms.