A 32-year-old woman presents with pruritic erythematous patches on the medial thighs and inguinal folds for 3 weeks. KOH mount shows branching septate hyphae. Which single clinical feature best distinguishes tinea cruris from tinea corporis?

## Distinguishing Tinea Cruris from Tinea Corporis ### Key Clinical Discriminator **Key Point:** Bilateral and symmetric involvement of intertriginous areas (groin, inner thighs) with characteristic **sparing of the scrotum** is the hallmark feature that distinguishes tinea cruris from tinea corporis. ### Comparative Features | Feature | Tinea Cruris | Tinea Corporis | |---------|--------------|----------------| | **Location** | Inguinal folds, medial thighs, intertriginous areas | Any non-intertriginous body surface (trunk, limbs, face) | | **Scrotum involvement** | Characteristically SPARED | May be involved if on body | | **Symmetry** | Bilateral and symmetric | Usually unilateral or asymmetric | | **Border pattern** | Raised, well-demarcated, advancing edge | Raised borders with central clearing | | **Maceration** | Often present due to moisture | Absent | | **Causative agents** | *T. rubrum* (most common), *E. floccosum* | *T. rubrum*, *M. canis*, *T. mentagrophytes* | ### Why Scrotum Sparing Matters **Clinical Pearl:** The scrotum is spared in tinea cruris because the thin, delicate scrotal skin lacks the keratinized layer that dermatophytes require for colonization. This is such a consistent finding that scrotal involvement should prompt you to reconsider the diagnosis. **High-Yield:** Tinea cruris is predominantly a disease of intertriginous surfaces (warm, moist areas) in males and females. The bilateral, symmetric pattern reflects the symmetry of groin anatomy and the uniform moisture environment. ### Mnemonic **SCROTAL SPARING = TINEA CRURIS** — If the scrotum is involved, think of a different dermatophyte infection or a secondary bacterial/candidal infection superimposed on tinea. ### Pathophysiology Tinea cruris thrives in the warm, humid intertriginous environment. The stratum corneum is thick in the groin but the scrotal epithelium is thin and lacks sufficient keratinized substrate for fungal invasion, making it a natural "immune zone" for dermatophytes. [cite:Fitzpatrick's Dermatology 9e Ch 183]