Which organism is well demonstrated by the colloidal iron stain?

• A. Cryptococcus neoformans
• B. Candida albicans
• C. Aspergillus fumigatus
• D. Histoplasma capsulatum

Answer: (A)

Colloidal iron stain targets acidic mucopolysaccharides, which are abundant in the polysaccharide capsule that encases Cryptococcus neoformans. When this stain is applied, the capsule binds the iron colloid and stands out as a distinct blue area around the yeast cell, making the capsule easily visible. This capsule is a defining feature of Cryptococcus and is not a prominent or defining feature of the other organisms listed. Candida albicans lacks a true capsule, so the stain won’t highlight a halo around its cells; Aspergillus fumigatus is a mold with hyphae rather than encapsulated yeast; Histoplasma capsulatum is a small intracellular yeast without a prominent capsule to emphasize with this stain. Therefore, the colloidal iron stain best demonstrates Cryptococcus neoformans.