A 12-year-old boy is brought to the clinic for progressively worsening blurry vision and generalized headaches that are worse in the morning. On further questioning, he is also revealed to have nausea, joint pain, itchy and peeling skin, dizziness, and irritability. Historically, he has been a healthy child and his family follows an organic diet with additional supplements that include 2 tablespoons of cod liver oil and a multivitamin tablet a day. On exam, his weight dropped from the 50th percentile a year before to the 10th percentile currently. He has oily skin and hair and cracking at the corners of his mouth. Chronic overdose of which of the following most likely caused his symptoms