What is the primary use of acridine orange in histopathology?

The primary use of acridine orange in histopathology is to demonstrate DNA and RNA. This fluorescent dye has the unique ability to bind to nucleic acids, and when it interacts with these substances, it emits fluorescence—typically emitting an orange color when bound to RNA and a green color when bound to double-stranded DNA. This property makes acridine orange especially valuable for identifying and visualizing cells and tissues rich in nucleic acids, which can be particularly useful in studies involving cell proliferation, apoptosis, and various pathological conditions like infections or cancers where there is a higher nucleic acid content.

In contrast, the other choices pertain to different staining techniques or purposes. For example, staining elastic fibers typically involves specific stains, such as Verhoeff's stain or orcein, rather than acridine orange. Staining plasma cells usually requires a different set of dyes that are capable of highlighting the cytoplasmic regions containing immunoglobulins, and evaluating fats involves specific lipid stains that are not compatible with acridine orange. Thus, the distinctive role of acridine orange in visualizing nucleic acids clearly establishes its primary use in the histopathology field.