A 28-year-old male presents with a 3-week history of an itchy rash on his trunk and arms. On examination, he has well-demarcated lesions with a characteristic ring-shaped appearance, as shown in the diagram marked **A**. The lesion demonstrates a raised, erythematous, scaly border with central clearing. KOH microscopy of scrapings from the active border reveals branching septate hyphae. Which of the following is the most likely causative organism?

Explanation

## Why Trichophyton mentagrophytes is right Trichophyton mentagrophytes (and related species such as T. indotineae) is the most common dermatophyte causing tinea corporis (ringworm of the body) in India and globally. The classic presentation of annular (ring-shaped) pruritic plaques with raised erythematous active border and central clearing, combined with KOH microscopy showing branching septate hyphae, is pathognomonic for dermatophyte infection. T. mentagrophytes is a zoophilic and anthropophilic dermatophyte that readily colonizes skin and causes the typical ringworm morphology. The diagnosis is confirmed by KOH microscopy (80–90% sensitivity) and culture on Sabouraud dextrose agar. (Murray 9e; Robbins 10e Ch 25) ## Why each distractor is wrong - **Trichophyton tonsurans**: This is the most common cause of tinea capitis (scalp ringworm) in India and North America, not tinea corporis. It typically presents with "black dot" alopecia or kerion (inflammatory boggy mass) on the scalp in children, not annular plaques on the trunk in an adult. - **Candida albicans**: Candida causes intertrigo (maceration and erosions in skin folds) and is not a dermatophyte. Candidal infections do not produce the characteristic annular plaques with raised borders and central clearing seen in this case. Additionally, KOH microscopy would show budding yeast and pseudohyphae, not branching septate hyphae. - **Malassezia furfur**: This lipophilic yeast causes tinea versicolor (pityriasis versicolor), which presents with hypopigmented or hyperpigmented macules and patches, not annular plaques. KOH microscopy shows "spaghetti and meatballs" appearance, not septate hyphae. **High-Yield:** Tinea corporis = annular plaques + raised active border + central clearing + KOH shows branching septate hyphae = dermatophyte (Trichophyton, Microsporum, or Epidermophyton); T. mentagrophytes is the most common cause in India. [cite: Murray 9e; Robbins 10e Ch 25]