You are answering a USMLE-style medical question. Question: A 34-year-old woman with poorly controlled Crohn disease comes to the physician because of a 2-week history of hair loss and a rash on her face. She has also noticed that food has recently tasted bland to her. She had to undergo segmental small bowel resection several times because of intestinal obstruction and fistula formation. Examination shows several bullous, erythematous perioral plaques. There are two well-circumscribed circular patches of hair loss on the scalp. A deficiency of which of the following is the most likely cause of this patient's condition? Options: A) Niacin B) Cobalamin C) Iron D) Zinc Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.