A 36-year-old male presents because his skin has been darkening recently. You notice that his skin has a dark, somewhat bronze color. Workup reveals signs of diabetes mellitus. His serum iron is found to be 1150 mg/dL, and his transferrin saturation is 98%. A liver biopsy is performed and reveals extensive deposits of hemosiderin in the hepatocytes and Kupffer cells. The mechanism most likely responsible for this constellation of findings is