A 52-year-old farmer from rural Maharashtra presents with a 10-day history of painful red eye with photophobia and blurred vision. Slit-lamp examination reveals a central corneal ulcer with raised, infiltrated borders, satellite lesions, and a hypopyon. The ulcer has a feathery margin and appears to be progressing despite topical antibiotics. Which investigation is most appropriate to confirm the suspected diagnosis?

Explanation

## Clinical Presentation Analysis The patient's presentation is highly suggestive of a **fungal corneal ulcer**: ### Key Clinical Features Pointing to Fungal Etiology | Feature | Significance | |---------|-------------| | **Feathery margin** | Characteristic of fungal ulcers; indicates filamentous growth pattern | | **Satellite lesions** | Pathognomonic for fungal infection; represents spread along nerve fibers | | **Raised, infiltrated borders** | Fungal ulcers tend to have more pronounced inflammatory response | | **Hypopyon** | Can occur in both bacterial and fungal, but persistence despite antibiotics suggests fungal | | **Slow progression (10 days)** | Fungal ulcers progress more slowly than bacterial ulcers | | **Rural setting, farmer** | Higher risk for environmental fungal exposure (Aspergillus, Fusarium, Candida) | | **Failure to respond to antibiotics** | Strong indicator of fungal rather than bacterial pathology | ## Investigation of Choice: KOH Mount and Fungal Culture **Key Point:** KOH (Potassium Hydroxide) mount is the **gold standard screening test** for fungal corneal ulcers because: 1. **Rapid turnaround** — Results available within 24 hours 2. **High sensitivity** — Dissolves bacterial and epithelial cells, leaving fungal elements visible under microscopy 3. **Cost-effective** — Inexpensive and widely available 4. **Morphologic identification** — Allows presumptive identification (septate vs. non-septate hyphae, budding yeast) **Fungal Culture** (Sabouraud's dextrose agar, blood agar): - Provides **definitive diagnosis** and species identification - Enables **antifungal susceptibility testing** - Takes 1–2 weeks but is essential for guiding therapy ### Why KOH Mount + Culture is Superior Here **High-Yield:** The combination of KOH mount (rapid presumptive diagnosis) + fungal culture (definitive identification and drug susceptibility) is the **standard diagnostic protocol** for suspected fungal keratitis [cite:Park 26e Ch 5]. **Clinical Pearl:** In resource-limited settings (common in rural India), KOH mount alone can guide empiric antifungal therapy while culture results are pending, reducing the risk of corneal perforation. ### Specimen Collection - **Corneal scraping** using a sterile spatula or Kimura spatula - Multiple scrapings from the ulcer margin (where organisms are most concentrated) - Avoid anesthetic contamination; use preservative-free drops - Inoculate directly onto culture media and prepare slides for microscopy