Medical Student

25 yr old man presented to the emergency room complaining about an ulcer in the lower limb. 15 days ago, while he was in the garden at night he felt a "sting" near his left knee but he didn't see any insect. The next couple of days he could see an erythematous area (wich was painful) , and he felt a rash. About 7 days ago the ulcer appeared on his left lower limb, the patient refer that "it has been growing since then" (see last picture). What would be the most likely ethiology of this injury based on the evolution and history of the patient?