A patient with chronic blepharitis develops a marginal corneal infiltrate separated from the limbus by a clear zone of cornea, with an epithelial defect on its superficial aspect. The underlying mechanism is:

• A Hypersensitivity reaction to staphylococcal exotoxins ✓
• B Direct invasion of corneal tissue by Staphylococcus aureus
• C Ischaemic infarction of the peripheral cornea
• D Autoimmune destruction mediated by anti-collagen antibodies

Explanation

Marginal catarrhal (phlyctenular-type) keratitis is a type III and IV hypersensitivity response to staphylococcal exotoxins and cell wall antigens delivered via lid margin secretions, not a true infection of the cornea. This explains the characteristic clear interval between the infiltrate and the limbus. Direct invasion would produce a central suppurative ulcer, and autoimmune collagen destruction describes Mooren ulcer or peripheral ulcerative keratitis.

Reference: Parsons' Diseases of the Eye, 23rd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.