A contact lens wearer presents with a painful red eye, photophobia, and a corneal ulcer that is ring-shaped and involves the mid-peripheral cornea. Pain is disproportionate to clinical signs. The most likely organism and the diagnostic test are:

• A Pseudomonas aeruginosa; corneal scraping for Gram stain and culture
• B Herpes simplex virus; corneal epithelial swab for viral culture
• C Aspergillus fumigatus; Sabouraud's agar culture
• D Acanthamoeba; corneal confocal microscopy or PCR of corneal scraping ✓

Explanation

Ring infiltrate in the mid-peripheral cornea with severe disproportionate pain in a contact lens wearer is the classic presentation of Acanthamoeba keratitis. The pain is intense due to perineural invasion by the organism. Diagnosis is by corneal confocal microscopy (showing cysts and trophozoites — double-walled cysts are pathognomonic) or corneal scraping sent for PCR, histology (PAS stain), or culture on non-nutrient agar with E. coli overlay. Treatment uses polyhexamethylene biguanide (PHMB) and propamidine isethionate (Brolene) — dual biguanide/diamidine therapy.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.