Q. A 45-year-old woman with a known history of iron-deficiency anemia presents to your primary care clinic for a follow-up visit 3 months after initiating treatment. She has been taking iron supplements as prescribed but reports limited improvement in her anemia symptoms. You decide to discuss factors that can enhance iron absorption to help manage her condition. In a patient with anemia, which of the following factors can enhance iron absorption in the duodenum to improve iron deficiency?