Contact dermatitis

A 17-year-old man comes to the clinic with a pruritic rash.He says that he noticed a vesicular rash on his arm after walking in the forest with his friends. His temperature is 37.1 ^oC, pulse is 80 /min, respiratory rate is 12/min, and blood pressure is 110/75 mmHg. Physical examination shows clear fluid filled vesicles on his arm in a linear pattern. He does not have stridor and air entry is good bilaterally. He is up-to-date on his immunizations and does not have any allergies. He does not smoke and does not drink alcohol. Which of the following is the most likely explanation for this patient's physical findings?