an ulcerated plaque on the hand of a young man

a 21-year old man presented with a 2-month history of a persistent ulcerated plaque on his right hand with no further lesion elsewhere. he was in contact with decorative aquarium in his hobbies associated with fish and water exposure. on physical examination there was a 2×2cm erythematous plaque with central ulceration on the thumb. there was no regional lymphadenopathy or similar lesion elsewhere and no specific finding of systemic involvement. he had no significant past medical history. the patient was treated with amoxycillin, however his condition failed to improve. for confirming the diagnosis a skin biopsy was ordered and a histopathological section of the lesion has been shown in figure 2. the specimen was also cultured in a specific medium. what is the diagnosis?