A spherule stage is seen in humans infected with what organism?
a. Blastomyces dermatitidis
b. Coccidioides immitis
c. Histoplasma capsulatum
d. Paracoccidioides brasiliensis

Verified step by step guidance

1. Understand the term 'spherule stage' in the context of fungal pathogens. A spherule is a large, thick-walled structure containing endospores, characteristic of certain dimorphic fungi during infection.
2. Recall the dimorphic fungi listed in the options and their unique morphological forms in human hosts: Blastomyces dermatitidis, Coccidioides immitis, Histoplasma capsulatum, and Paracoccidioides brasiliensis.
3. Identify which of these fungi forms spherules in human tissue. Specifically, Coccidioides immitis is known to produce spherules filled with endospores during infection.
4. Contrast this with other fungi: Blastomyces dermatitidis forms broad-based budding yeast, Histoplasma capsulatum forms small intracellular yeast, and Paracoccidioides brasiliensis forms multiple budding yeast resembling a 'pilot wheel'.
5. Conclude that the presence of spherules in human infection is a hallmark of Coccidioides immitis.

Key Concepts

Spherule Stage in Fungal Life Cycles

The spherule is a specialized, thick-walled structure formed by certain fungi during infection in humans. It contains endospores and is a key diagnostic feature, especially in tissue samples. Understanding which fungi produce spherules helps identify the causative agent of infection.

Coccidioides immitis and Coccidioidomycosis

Coccidioides immitis is a dimorphic fungus that causes coccidioidomycosis (Valley fever). In the human host, it forms spherules filled with endospores, which rupture to release new infectious particles. This spherule stage is unique to Coccidioides among the listed fungi.

Differentiation of Dimorphic Fungi

Dimorphic fungi can exist in mold or yeast forms depending on environmental conditions. Each species has distinct morphological features in the host, such as yeast, spherules, or intracellular forms, which aid in diagnosis. Recognizing these differences is essential for identifying the infecting organism.