A patient with chronic blepharitis develops a peripheral oval corneal ulcer separated from the limbus by a clear zone of cornea, with a mild anterior chamber reaction. The ulcer has a white, soft infiltrate and the eye is relatively comfortable. What is the underlying mechanism?

• A Direct bacterial invasion of corneal stroma by Staphylococcus
• B Fungal superinfection from lid flora
• C Autoimmune vasculitis of limbal vessels
• D Type IV hypersensitivity reaction to Staphylococcal exotoxins ✓

Explanation

Staphylococcal marginal keratitis (catarrhal ulcer) is a type IV hypersensitivity reaction to exotoxins from Staphylococcus aureus at the lid margin, not direct infection. The clear limbal zone, soft infiltrate, and relatively comfortable eye distinguish it from infectious ulcers. Autoimmune vasculitis describes Mooren's ulcer or PUK with systemic disease.

Reference: Basic and Clinical Science Course, Section 8: External Disease and Cornea, AAO 2023-2024 ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.