A 28-year-old male athlete presents with scaly, pruritic patches on the dorsal feet and between the toes for 2 months. Fungal culture grows *Trichophyton mentagrophytes*. Which feature most reliably distinguishes tinea pedis from tinea corporis caused by the same organism?

## Distinguishing Tinea Pedis from Tinea Corporis (Same Organism) ### Site-Specific Morphology as the Discriminator **Key Point:** When the same dermatophyte (*T. mentagrophytes*, *T. rubrum*) infects different body sites, the **morphology and presentation differ dramatically based on local skin physiology**. Maceration, fissuring, and erosion in the intertriginous toe spaces are pathognomonic for tinea pedis and do not occur in tinea corporis caused by the same organism. ### Comparative Table: Same Organism, Different Sites | Feature | Tinea Pedis | Tinea Corporis (same agent) | |---------|-------------|-----------------------------| | **Location** | Plantar surface, toe webs, dorsal feet | Trunk, limbs, non-intertriginous areas | | **Maceration & fissuring** | CHARACTERISTIC (especially toe webs) | Absent | | **Erosion/denudation** | Present in interdigital form | Absent | | **Scaling pattern** | Hyperkeratotic, diffuse; or vesiculobullous | Well-demarcated plaques with raised borders | | **Border definition** | Often ill-defined in hyperkeratotic form | Sharp, raised, advancing border | | **Moisture environment** | Perpetually moist (occlusion by footwear) | Variable | | **Causative agents** | *T. mentagrophytes*, *T. rubrum*, *E. floccosum* | *T. mentagrophytes*, *T. rubrum*, *M. canis* | ### Why Maceration Occurs in Tinea Pedis **Clinical Pearl:** The interdigital toe spaces are the most occluded, moisture-rich microenvironment on the body. Constant sweating, poor ventilation, and tight footwear create a perpetually macerated state. The fungus produces proteolytic enzymes that break down the already-softened stratum corneum, leading to erosions and fissures. This does NOT happen on the dry, well-ventilated trunk (tinea corporis). **High-Yield:** Tinea pedis has three clinical variants, each with a different morphology: 1. **Interdigital (most common):** Maceration, fissuring, erosion between 4th and 5th toes. 2. **Hyperkeratotic (moccasin type):** Diffuse scaling on sole and heel; no maceration. 3. **Vesiculobullous:** Vesicles and pustules on arch and sides; inflammatory. Tinea corporis, by contrast, almost always presents as a **single annular plaque** with central clearing and a raised, advancing border—the classic "ringworm" appearance. ### Mnemonic **FOOT FISSURES = TINEA PEDIS** — Maceration and fissuring in toe webs are the site-specific signature of tinea pedis and are virtually never seen in tinea corporis, even when caused by the same organism. ### Pathophysiology The stratum corneum in the toe webs is thin and constantly exposed to moisture. Dermatophytes produce keratinolytic enzymes that, combined with the macerated state, cause breakdown of the epidermal barrier. On the trunk, the stratum corneum is thicker, drier, and better able to resist enzymatic degradation, so the infection manifests as a discrete, well-demarcated plaque instead. [cite:Fitzpatrick's Dermatology 9e Ch 183]