Fluorescein stain is primarily used to diagnose what condition?

Fluorescein stain is primarily used to diagnose corneal ulcers because it highlights defects in the corneal epithelium where the stain will penetrate. When fluorescein dye is applied to the eye, it will only stain areas where the corneal covering is damaged, allowing for easy visualization of ulcerations or abrasions in the cornea. This diagnostic tool is particularly valuable in veterinary medicine to assess the integrity of the cornea and identify any lesions that may be causing discomfort or vision issues in animals.

In contrast, conditions like cataracts, conjunctivitis, and retinal detachment do not primarily require fluorescein staining for diagnosis. Cataracts are generally assessed through a physical examination and other imaging techniques, conjunctivitis may involve evaluation of redness and discharge, and retinal detachment is typically diagnosed via specialized imaging like ultrasound or direct examination of the retina, not through fluorescein staining.