A 22-year-old woman presents with a 4-month history of intensely pruritic, erythematous rash on her earlobes and lateral neck that worsens during the day and improves when she removes her earrings. She recently started wearing inexpensive costume jewelry. She has no fever, wheezing, or facial swelling. Exam shows well-demarcated, scaly, erythematous plaques with excoriations in areas contacting the earrings. Which of the following is the most likely causative agent for this dermatitis?

(Assume the dermatitis is allergic rather than irritant.)