## Diagnosis: Tinea Cruris (Jock Itch) ### Clinical Presentation **Key Point:** Tinea cruris presents with well-demarcated erythematous plaques with a raised, advancing border and central clearing on the groin and inner thighs, classically sparing the scrotum (unlike candidiasis). ### Causative Organism **High-Yield:** *Trichophyton rubrum* is the most common cause of tinea cruris worldwide and in India, accounting for 70–80% of cases. It is an anthropophilic dermatophyte. ### Mycological Features - **Branching septate hyphae** on KOH mount (diagnostic for dermatophytes) - Grows on Sabouraud dextrose agar - T. rubrum shows slow growth, white to cream-colored colonies with red pigment on the reverse side ### Differential Considerations | Feature | T. rubrum | T. mentagrophytes | Candida albicans | M. furfur | |---------|-----------|-------------------|------------------|----------| | **Morphology** | Septate hyphae | Septate hyphae | Yeast + pseudohyphae | Yeast only | | **Common site** | Groin, feet, nails | Feet, nails, glabrous skin | Intertriginous areas | Trunk (pityriasis) | | **Scrotum involvement** | Rare | Rare | Common | No | | **Prevalence in India** | Most common | Less common | Intertriginous only | Pityriasis versicolor | **Clinical Pearl:** The sparing of the scrotum is a key clinical clue that excludes candidiasis, which commonly involves the scrotum and produces satellite pustules. ### Why T. rubrum? - Most common dermatophyte in India and globally - Causes tinea cruris, tinea corporis, tinea pedis, and onychomycosis - Anthropophilic (human-to-human transmission) - Responds well to topical azoles and allylamines **Mnemonic:** **T. rubrum = TRue dermatophyte** — the most common cause of dermatophytosis in humans. 
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