Fluorescein is used to observe all of the following except:

Fluorescein is a dye commonly used in ophthalmology to assess various conditions of the eye, particularly when evaluating the status of the cornea and the fit of contact lenses. When fluorescein is applied to the eye, it highlights areas of concern such as corneal abrasions, dryness, or irregularities in the contact lens fit by staining them, which can be observed under a blue light.

In the context of the options, while fluorescein is effective for evaluating contact lens fit, corneal dry spots, and punctate staining—which typically indicates areas of epithelial damage or desiccation—it is not specifically used for corneal striae. Corneal striae are lines or folds in the corneal tissue that can occur due to various factors, such as elevation of the cornea from increased pressure or other structural changes. These striae are not indicative of surface issues that fluorescein would stain, and they typically do not require the same evaluation method.

Thus, the use of fluorescein is not applicable for observing corneal striae directly, rendering this option the one for which fluorescein is not utilized.