A concerned mother brings her 7-day-old son to his pediatrician after he develops a rash. Examination reveals a cluster of erythematous vesicles localized to the child's mouth. The child's vital signs are all within normal limits, and aside from the rash and mild irritability he appears healthy. A sample of tissue is taken from the base of a ruptured vesicle and subjected to the Tzanck test . The child is treated with the most commonly prescribed agent for this condition. Which of the following is a serious adverse effect associated with this drug?