Which mode of slit-lamp illumination is used to view early stages of microcystic edema?

The correct choice for viewing early stages of microcystic edema is retroillumination. This technique utilizes light that is reflected from the anterior structures of the eye, such as the cornea, and allows the examiner to detect subtle changes in the tissues that may not be visible with direct illumination. During retroillumination, the light source is positioned behind the area of interest, illuminating it from an angle. This causes the edema to appear as a shadow or a disruption in the normal light reflection pattern, making it easier to identify.

In contrast, direct illumination shines light directly onto the eye, which may not highlight the subtle changes associated with microcystic edema as effectively. The parallelpiped technique involves using a narrow beam of light to examine the eye's structures in cross-section, but it may not reveal the same level of detail when looking for microcystic edema. Oscillation refers to a dynamic light technique that can help assess movement but is not typically used for identifying microcystic edema specifically. Overall, retroillumination is particularly effective in detecting the early, subtle signs of this condition.