= Solution
\b[A ring-shaped patch is a clinical shape, not one histological disease.] The historical category included several named scalp and skin affections whose relationship was disputed. In particular, old claims that <alopecia areata> was a fungal disease cannot be carried into modern <pathology>. A contemporary review actually records competing classifications of favus, scalp ringworm, circinate eruptions, beard disease and patchy baldness, and disagreement about whether their fungi were identical.<On the Parasitic Affections of the Skin (review, 1864)>[Discussion of proposed parasites of favus, herpes tonsurans, herpes circinatus, sycosis, tinea decalvans and pityriasis versicolor]{ref}
The microscopic distinctions should be organized by the structure infected or injured:
* \b[Ordinary scalp <ringworm>, or <tinea capitis>.] The <fungus> invades keratinized <hair> and adjacent surface material. <Hyphae> and <arthroconidia> can occur within the shaft, the <endothrix> pattern, or around its exterior, the <ectothrix> pattern. The weakened shaft breaks, producing short stumps or dark dots. Inflammation varies from limited scale to a severe follicular reaction. Modern species cannot reliably be assigned from the shape of the bald patch alone.<Tinea capitis by DermNet>[Classification by invasion of the hair shaft; clinical variants and microscopy]{ref}
* \b[<Favus>.] The characteristic yellow cups, or <scutula>, consist of a mass of fungal filaments and keratinous debris around a <hair follicle>. Hyphae may also involve the hair. Longstanding destructive disease can scar the follicle and cause permanent <hair loss>. The historical name of its organism was Achorion schoenleinii; the usual modern designation is <Trichophyton schoenleinii>. Those names describe the history of classification, not two different appearances inferred from an unseen specimen.<Favus in StatPearls>[Introduction and Etiology, scutula and fungal cause]{ref}
* \b[<Tinea corporis> and fungal beard disease, <tinea barbae>.] On smooth <skin>, superficial hyphae occupy the <stratum corneum>, with variable epidermal reaction; the advancing margin can be the most productive site for scrapings. In beard infection the hair-bearing <hair follicles> are involved, and the reaction may be follicular, pustular or granulomatous. The historical word <sycosis> also covered nonfungal follicular inflammation, so its name alone does not establish a dermatophyte.<Tinea corporis pathology by DermNet>[Histology and special stains]{ref}<Tinea barbae by DermNet>[Microscopy of scrapings and hairs; inflammatory follicular disease]{ref}
* \b[<Pityriasis versicolor>.] This is associated with <Malassezia>, not the usual dermatophytes of true <ringworm>. Numerous rounded yeast forms and short fungal filaments occur predominantly in the <stratum corneum>; a hair shaft filled by arthroconidia is not the defining lesion. This explains why the older name “tinea versicolor” should not imply identity with scalp <ringworm>.<Pityriasis versicolor pathology by DermNet>[Histology, yeast and short hyphae in the stratum corneum]{ref}
* \b[<Alopecia areata>, historically entangled with “tinea decalvans”.] The modern lesion is immune-mediated disturbance of hair follicles, with characteristic lymphocytic inflammation around and sometimes within the bulb in active disease and changes in the hair-growth cycle. The inflammatory infiltrate may be inconspicuous in chronic disease. Finding short or altered hairs is not proof of fungi.<S3 guideline diagnostics and therapy of alopecia areata, Part 1>[Histopathological diagnostics, active peribulbar inflammation and chronic-stage variation]{ref}
Annular <eczema> or <psoriasis> supplies a further clinical mimic: an inflammatory epidermal pattern alone is not evidence of an invading <fungus>. For an actual microscopic investigation, collect scale from an active margin and affected hairs when hair is involved, mount the material on a <microscope slide> with <potassium hydroxide> to clear keratin, and examine under a <microscope>. A modern <biopsy> can be examined with <Periodic acid-Schiff staining> to highlight fungal structures. Distribution within the surface layer, hair shaft, follicle or bulb matters as much as the presence of particles. <Fungal culture> or molecular identification can resolve species when morphology cannot.<Investigation of Dermatological Specimens for Superficial Mycoses by the UK Standards for Microbiology Investigations>[B 39, issue 3.2, Microscopy and specimen examination]{ref} \b[Distinguish the organism, its anatomical location and the host reaction; do not diagnose all circular lesions as one infection.]
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