An archaeologist working in the deserts of Arizona develops fever, cough and erythema nodosum. Lung biopsy shows thick-walled spherules measuring 20 to 80 micrometres filled with small endospores. Which form of this fungus is infectious to humans?

• A Yeasts growing in tissue
• B Spherules released from lesions
• C Microconidia from mold colonies
• D Arthrospores from soil hyphae ✓

Explanation

Coccidioides immitis and posadasii grow in desert soil as moulds that fragment into barrel-shaped arthrospores, which are the infective particles inhaled from dust. Inside the host they enlarge into spherules that rupture to release endospores, each capable of forming a new spherule. The tissue form is not contagious and person-to-person transmission does not occur, so option B is the tempting distractor that must be excluded. Erythema nodosum indicates a good cellular immune response and carries a better prognosis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.