The India ink stain is used as a presumptive test for the presence of which organism?

The India ink stain is commonly used as a presumptive test for the presence of Cryptococcus neoformans, particularly in cerebrospinal fluid (CSF) samples. This organism is a type of encapsulated yeast, and the India ink preparation allows for the visualization of the thick polysaccharide capsule surrounding the yeast cells. When CSF mixed with India ink is examined under a microscope, the organisms appear as round yeast cells with a distinct clear halo or "halo effect" around them, which represents the capsule. This characteristic appearance is crucial for the identification of Cryptococcus neoformans, particularly in the context of diagnosing conditions such as cryptococcal meningitis. Other options listed pertain to different organisms or contexts where the India ink preparation is not applicable or relevant for definitive diagnosis.