American Board of Internal Medicine (ABIM) Certification Practice Exam

What is typically seen in a blood smear of a patient with histoplasmosis?

1. Large yeast forms

2. Small yeast forms within neutrophils

3. Hyphae structures

4. Parasitic infections

The correct answer is: Small yeast forms within neutrophils

In the context of histoplasmosis, the presence of small yeast forms within neutrophils is a characteristic finding in blood smears. Histoplasmosis is caused by the fungus Histoplasma capsulatum, which can be inhaled or enter the body through the respiratory tract. Once in the bloodstream, the organism can be phagocytosed by neutrophils and macrophages, leading to the identification of these small yeast forms, typically measuring 2-5 micrometers. The small yeast forms may not be easily distinguished in all cases, particularly in extrathoracic dissemination, but their presence within neutrophils serves as a significant indicator of infection. This feature helps to differentiate histoplasmosis from other fungal infections. In contrast, large yeast forms are associated with other infections like blastomycosis, and hyphal structures are typically indicative of invasive fungal infections such as aspergillosis or candidiasis, neither of which aligns with histoplasmosis. Parasitic infections do not pertain to histoplasmosis, as this condition is a mycotic disease rather than a parasitic one, further clarifying the unique characteristics of the pathogens involved in different infections.