= Solution
\b[The historical name <pityriasis> grouped finely scaling eruptions; its varieties were not all one <fungal infection>.] A useful period list is simple or scalp <pityriasis>, <pityriasis rubra>, <pityriasis versicolor> and <pityriasis nigra>. The names describe distribution or colour, and classifications differed. In particular, “nigra” was sometimes a very dark form of “versicolor,” sometimes a pigmentation disorder with little true scaling. It cannot safely be translated into one present species of <fungus>.<The Practice of Medicine by Robley Dunglison (1844)>[Pityriasis: capitis, rubra, versicolor and nigra; limits of the nigra category]{ref}
Simple <pityriasis>, often called <pityriasis capitis> on the scalp, gives loose, whitish, bran-like scales with little or variable redness. <Microscopy> of a scale principally shows flattened keratinised <epithelial cells> and debris. An oily or inflamed scalp can also contain organisms normally living there; this does not make every case identical to <pityriasis versicolor>. Present descriptions of <dandruff> and <seborrhoeic dermatitis> explain many such appearances, but the historical label cannot retrospectively specify the cause of each case.<Seborrhoeic Dermatitis by DermNet>[Clinical features and relationship to seborrhoeic dermatitis]{ref}
<Pityriasis rubra> denoted a conspicuously red, scaling eruption, sometimes extensive. Detached scales are again principally epidermal material. A modern <skin biopsy> may show an inflammatory pattern, but there is no single microscopical organism that defines the historical category. It must not automatically be renamed <pityriasis rubra pilaris>, a distinct follicular disorder. An extensive red and peeling <skin> is a pattern requiring causal diagnosis, rather than proof of one infection.<Erythroderma by DermNet>[Definition, causes and nonspecific biopsy findings]{ref}
<Pityriasis versicolor> produces irregular pale, yellowish-brown or brown patches, commonly on the upper <anatomical trunk>, with a fine surface scale. Here the <microscopy> is especially discriminating: a superficial scraping, cleared with <potassium hydroxide>, shows clusters of round yeast forms and short curved <hyphae> within the <stratum corneum>. Modern classification places the organisms in <Malassezia>. The older “Microsporon furfur” terminology should not be read as identification of a modern <Microsporum> dermatophyte. On a <histological section> the organisms occupy the horny surface layer and are made conspicuous by suitable <fungal stains>.<Pityriasis versicolor pathology by DermNet>[Histology and special stains: yeasts and short hyphae in the stratum corneum]{ref}
<Pityriasis nigra> calls first for verification of what is actually being described. If it is dark <pityriasis versicolor>, the same superficial yeast and <hyphae> may be found; if the lesion is pigmentation without a fungal eruption, this finding cannot be assumed. A contemporary discussion explicitly recorded the overlap of nigra and versicolor and disputes over the significance of the supposed parasite.<On the Phytopathology of the Skin (review, 1864)>[pp. 208–209 and 218–219: versicolor, nigra and conflicting fungal interpretations]{ref} Thus \b[epidermal scales are common to the description, but demonstrable fungal structures particularly distinguish <pityriasis versicolor>]. Examine actual scrapings with a <optical microscope>, compare several fields and use appropriate controls before attributing a different scaling disorder to a <fungus>.
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