**Core Concept:** Corneal erosions and ulcers are conditions involving damage or breakdown of the corneal epithelial layer, leading to potential visual disturbances. Corneal staining is a method used to visualize these lesions and assess the extent of epithelial damage.

**Why the Correct Answer is Right:** Fluorescein is a fluorescent dye commonly used for staining the ocular surface, specifically the cornea. When applied to the eye, fluorescein binds to the superficial corneal epithelial cells, highlighting the damaged areas due to corneal erosions or ulcers. In the presence of intact epithelium, fluorescein remains on the surface, forming a bright, greenish-yellow stain. In areas with epithelial defects, the dye penetrates the stroma, resulting in a distinct, bright-green fluorescence.

**Why Each Wrong Option is Incorrect:**

A. Cyanine dyes (Option A) like Rose Bengal are not commonly used for corneal staining as they do not specifically highlight the corneal epithelium and are less sensitive than fluorescein.
B. Calcofluor White (Option B) is a fungal stain and does not specifically target the corneal epithelium.
C. Lissamine Green (Option C) is a dye used for staining the conjunctiva, not the cornea.
D. Fluorescein sodium (Option D) is a fluorescent dye used for staining the corneal epithelium and is a common choice for ocular surface staining.

**Clinical Pearl:** Fluorescein staining is essential for assessing corneal health, particularly during an ocular examination. It aids in identifying epithelial defects, such as those seen in corneal erosions or ulcers, allowing for early diagnosis and appropriate management.