Scale
Scale is visible flaking of the outermost layer of the skin, produced when cells are shed in clumps rather than individually. Its character differs between conditions, and clinicians describe how it looks as well as noting that it is present.
Skin is shed continuously and invisibly. Cells reaching the surface of the outermost layer detach one at a time, in quantities nobody notices. Scale is what happens when that process goes wrong and cells come away in clumps large enough to see.
Two routes lead there. Cells may be produced faster than they can mature, arriving at the surface before they are ready to separate cleanly, which is what happens in psoriasis. Or the outer layer may be disturbed for other reasons, including dryness, inflammation or an organism feeding on the keratin it is made of.
What clinicians record is not simply that scale is present but what it is like, because the character discriminates between conditions. Silvery and adherent, coming away with resistance, points differently from greasy and yellowish. Fine and powdery differs from thick and plate-like.
The distinction between silvery adherent scale and greasy yellowish scale is one of the standard observations separating psoriasis from seborrhoeic dermatitis on the scalp, where the two can otherwise resemble each other closely enough that a hedged term exists for the overlap.
Where scale sits also matters, since it usually accompanies something else. Scale on a raised well-demarcated area is a different observation from scale on flat skin, which is why the word rarely appears on its own in a description.
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