Which laboratory test is most commonly used to diagnose dermatophyte skin infections by demonstrating hyphae?

• A. Gram stain
• B. Calcofluor white staining
• C. KOH preparation of skin scrapings
• D. Fungal culture

Answer: (C)

Direct visualization of fungal elements in skin scrapings is the quickest way to diagnose dermatophyte infections. Potassium hydroxide loosens and clears keratin in the specimen, leaving the fungal hyphae visible under a standard light microscope. This simple, rapid, and inexpensive preparation makes it the most commonly used initial test for suspected tinea infections, allowing clinicians to confirm the presence of hyphae without waiting for culture. Gram stain is not optimized for fungi and often misses hyphae; fungal culture can identify the species but takes longer; calcofluor white staining is sensitive and can aid detection but requires fluorescence microscopy and is not as universally used as the KOH prep.