Which order of fungi is responsible for mucormycosis and what is a key histologic feature?

• A. Ascomycota; septate hyphae with acute-angle branching
• B. Basidiomycota; yeast-like cells with budding
• C. Mucorales; broad, non-septate hyphae with irregular widths and right-angle branching
• D. Zygomycota; hyphae with septate walls

Answer: (C)

The main idea being tested is recognizing mucormycosis by its fungal cause and its distinctive tissue appearance. The fungi that cause mucormycosis belong to the order Mucorales. Histologically, they form broad, ribbon-like hyphae that are non-septate (or sparsely septate) and show irregular widths. These hyphae tend to branch at wide angles, often at right angles. This combination—broad, non-septate hyphae with irregular widths and right-angle branching—is the hallmark that sets mucormycosis apart from other fungal infections.

In contrast, fungi like Aspergillus have septate hyphae with acute-angle (often about 45 degrees) branching, and yeasts or yeast-like forms (as seen with Basidiomycota or Candida) do not present as broad, non-septate hyphae. While taxonomy can be nuanced, the key histologic feature to identify here is the broad, non-septate hyphae with right-angle branching characteristic of Mucorales, which aligns with mucormycosis’s angioinvasive behavior in susceptible patients.