You see a 32 year old Caucasian farmer who presents with a 2 day history of a painful lesion on his index finger. He denies any trauma but admits to often having cuts and grazes on his hands from manual labour. He is well in himself and denies any fever or rash elsewhere. He has no other past medical history and no recent travel history. On examination there is a 2 cm firm red-blue papule which is tender to palpate. His observations are normal. What is the most likely diagnosis?