A 30-year-old farmer has a slowly enlarging annular plaque on the forearm with central clearing and an active scaly border. What is the most appropriate confirmatory investigation?
- A KOH mount of scales from the active border ✓
- B Skin biopsy showing granulomatous inflammation
- C Wood's lamp examination of the lesion
- D Tzanck smear from the edge
Correct answer: A. KOH mount of scales from the active border
Explanation
The classic annular plaque with an active peripheral edge and central clearing is tinea corporis, and the confirmatory test is direct microscopy of scales scraped from the advancing edge mounted in 10 to 20 percent KOH, which shows septate branching hyphae. Scales must come from the active border because the centre often clears and may be falsely negative. Wood's lamp is useful mainly for some tinea capitis and erythrasma, not routine tinea corporis.
Reference: Fitzpatrick's Dermatology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.