In a 28-year-old female with tinea corporis, the most common site of involvement is which of the following?

Explanation

## Most Common Site of Tinea Corporis **Key Point:** The extensor surfaces of the trunk and limbs (especially the forearms, shins, and outer aspects of the arms) are the most common sites of tinea corporis, accounting for the majority of cases. ### Anatomical Distribution of Tinea Corporis | Site | Frequency | Characteristics | Organisms | |---|---|---|---| | **Extensor surfaces (trunk, limbs)** | 60–70% | Well-demarcated annular lesions; less maceration | T. rubrum, T. mentagrophytes | | Flexural areas (axillae, groin, inframammary) | 15–20% | Macerated, erythematous; may coalesce | T. rubrum, E. floccosum | | Palms and soles | 10–15% | Hyperkeratotic, scaly; "moccasin" pattern | T. rubrum, T. mentagrophytes | | Scalp and nails | <5% | Tinea capitis (children); onychomycosis (adults) | T. tonsurans, T. rubrum | **High-Yield:** Tinea corporis classically presents as a **ring-shaped (annular) lesion with a raised, erythematous border and central clearing** on extensor surfaces. This is why it is sometimes called "ringworm," though it is not caused by a worm. ### Clinical Pearl **Tip:** The **"two-feet-one-hand" rule** helps distinguish dermatophyte infections from candidiasis: - Dermatophytes → typically **asymmetric** involvement (e.g., both feet + one hand) - Candidiasis → typically **symmetric** and in flexural areas Extensor surfaces are drier and less occluded, making them less ideal for fungal growth; however, dermatophytes (especially T. rubrum) still prefer these areas because they are exposed and easily traumatized, allowing fungal entry. **Warning:** Do not confuse tinea corporis on flexural areas with candidal intertrigo. Candidiasis causes maceration, satellite pustules, and is more common in obese or diabetic patients. Dermatophytes cause well-demarcated annular lesions with minimal maceration. ### Mnemonic **EXTEND** — **E**xtensor surfaces are the **EXTEND**ed (most common) site of tinea corporis. [cite:Robbins 10e Ch 25]