What substance is blocked due to focal hyperpigmentation on fluorescein angiography (FA)?

The correct answer relates to the mechanism of fluorescein angiography (FA) and how focal hyperpigmentation affects the visualization of certain structures in the eye. In fluorescein angiography, fluorescein dye is injected into the bloodstream and enhances the visualization of blood vessels in the retina and choroid. When there is focal hyperpigmentation—such as that seen in conditions like age-related macular degeneration (AMD)—the areas of increased pigmentation absorb the fluorescein dye, preventing it from illuminating these regions. This leads to a blocking of the fluorescein signal on the angiogram, making those pigmented spots appear darker compared to surrounding areas where fluorescein can freely pass and highlight the blood vessels.

This phenomenon specifically demonstrates how focal hyperpigmentation obstructs the visualization of fluorescein in affected areas rather than influencing the underlying structures directly, such as the choroid, drusen, or lipofuscin. In contrast, drusen and lipofuscin are entities found within the retina and retinal pigment epithelium but do not directly result in a blockage of fluorescein itself on imaging. Thus, understanding this interaction is critical in interpreting FA results in patients suspected of having retinal pathologies associated with increased