Case Number : Case 2010 - 19 Feb 2018 Posted By: Limin Yu

Keratin-filled crater, buttressing, pseudoepitheliomatous hyperplasia with glassy keratinocytes, hypergranulosis and dense lichenoid infiltrate of lymphocytes with a few eosinophils indicate a keratoacanthoma

Hypertrophic Lichen Planus.

I had thought of hypertrophic lichen planus like Vincenzo but decided against it because 1. The site - very unusual on the arm .2. epidermal hyperplasia is too massive and narrow. 3, keratin plug in the centre is too big and the crater has buttressing

Agree follicular pseudoepitheliomatous hyperplasia with a background follicular lichenoid infiltrate. Certainly hypertrophic lichen plano-pilaris would be top of the list and needs CPC. You don't usually see a solitary lichenplano-pilaris-like (lichenoid) keratosis but I suppose it could happen. KA are rarely this small and usually reach deeply and highly invasively into the deep reticular dermis to the level of the sweat gland coils in the proliferative stage.  Weedon did however draw attention to occasional so-called abortive KA but they are small and rather rounded in low power, typically infiltrative on close inspection of he borders that are rather overwhelmed by the inflammatory (predominantly lymphocytic) infiltrate, innumerable colloid bodies and striking individual cell necrosis in my small experience of them.

Thanks Anil and Richard for your very useful comments. 

Get level.

May be a tip of the iceberg of SCC.

This is getting interesting! Too small for a KA, too big for hypertrophic lichen planus and too innocuous for a SCC!

All the comments are very insightful.  I had the exactly same thoughts and differential diagnoses as you.  The clinical diagnosis is hypertrophic LP, so it was signed out as HLP. 

You can get very large clinical lesions with PEH including HLP.