Short tapered hairs that can be seen at the edge of alopecia areata patches.
Exclamation mark hairs are broken, short hairs that are narrower near the scalp and thicker at the free end. They are classically associated with active alopecia areata, especially around the margin of a patch. Their presence supports the diagnosis but must be interpreted in context, because other forms of breakage can also create short hairs. Trichoscopy helps distinguish exclamation mark hairs from broken hairs due to pulling, traction, fungal infection or hair shaft fragility.
BAD alopecia areata: https://www.bad.org.uk/pils/alopecia-areata ; DermNet trichoscopy: https://dermnetnz.org/topics/trichoscopy