Q. A 35-year-old male presents to your clinic with high-grade fever, fatigue, and sudden weight loss. On examination, you notice petechiae on his skin, gum hypertrophy, and an enlarged spleen. Laboratory findings indicate a marked increase in immature myeloid cells. Further cytogenetic analysis reveals the presence of the t(15;17) translocation. As part of his treatment, you consider starting him on a specific vitamin in addition to chemotherapy. Meanwhile, a 5-year-old child is brought to the clinic with high fever, cough, coryza, and a maculopapular rash that began on the face and is spreading downward. She has also developed a fine white rash inside her mouth. As part of her treatment, you consider the same vitamin. In which of the following diseases is the particular vitamin indicated?