A 32-year-old man presents with pruritic erythematous patches on the groin and inner thighs for 3 weeks. KOH mount shows septate hyphae. Another patient with similar clinical presentation has lesions confined to the plantar surface and sides of the feet with a 'moccasin' distribution. Which feature best distinguishes tinea pedis from tinea cruris?

Explanation

## Clinical Distinction Between Tinea Pedis and Tinea Cruris ### Key Anatomical Differences | Feature | Tinea Pedis | Tinea Cruris | |---------|-------------|-------------| | **Primary site** | Plantar surface, interdigital spaces, lateral borders | Inguinal folds, inner thighs, scrotum (in males) | | **Distribution pattern** | Moccasin type (sole + lateral borders) or interdigital | Bilateral, symmetrical, respects midline | | **Inguinal fold involvement** | Rare, typically spares folds | Characteristic involvement of folds | | **Maceration** | Common in interdigital type | May occur in moist environments | | **Causative agents** | T. rubrum, T. mentagrophytes, E. floccosum | T. rubrum (most common), T. mentagrophytes | **Key Point:** Tinea pedis characteristically involves the **plantar surface in a moccasin distribution** (sole + lateral borders), whereas tinea cruris involves the **inguinal folds and inner thighs while respecting the midline**. This anatomical distinction is the most reliable clinical discriminator. ### Why This Matters Clinically **Clinical Pearl:** The moccasin distribution in tinea pedis is pathognomonic — it represents the most common presentation and is rarely seen in tinea cruris. Conversely, inguinal fold involvement is virtually diagnostic of tinea cruris and absent in tinea pedis. **High-Yield:** Both conditions show septate hyphae on KOH mount (not discriminatory), both are commonly caused by T. rubrum, and both may show maceration depending on moisture and friction. The **site of involvement** is the most reliable distinguishing feature. ### Pathophysiology of Site Predilection 1. **Tinea pedis**: Feet provide warm, moist, occluded environment (especially between toes and under footwear) 2. **Tinea cruris**: Inguinal region provides heat, moisture, and friction from clothing and skin-to-skin contact **Mnemonic:** **FOOT** for tinea **P**edis (Plantar, Plantar-lateral, Plantar-sole distribution) vs. **FOLD** for tinea **C**ruris (Crural folds, inguinal involvement). [cite:Fitzpatrick's Dermatology in General Medicine Ch 133]