History: 4163tf 5 week history of these mildly itchy plaques on the lower legs and thighs . Came up after walking trip when small ?thorns or plant spikes penetrated her skin. Settle with oral steroids but flare when dose reduced. Looks a dermal infiltrative process. DD Erythema multiforme, dermal hypersensitivity process, Sweet's syndrome 2 biopsies knee and thigh

Description: Lichenoid reaction

Comments: The changes present are those of erythema multiforme. There is a lichenoid reaction with basal epidermal vacuolar change and Civatte body formation. There is a superficial, mid and deep dermal perivascular and perifollicular infiltrate of lymphocytes, histiocytes and some eosinophils. There is some obscuring of the dermo-epidermal interface by this lymphocytic infiltrate. In addition there is moderate subepidermal oedema. Some Civatte bodies extend above the basal level of the epidermis to the mid level of the epidermis. The aetiology in this patient is likely to be related to direct plant contact.