A 52-year-old man with rheumatoid arthritis on chronic anti-TNF therapy presents with fatigue and dyspnea. Laboratory findings show: hemoglobin 9.2 g/dL, serum iron 35 µg/dL (low), transferrin saturation 18% (low), but ferritin 280 ng/mL (elevated). The structure marked **D** in the diagram is abnormally elevated due to chronic inflammatory cytokine signaling. Which of the following best explains the mechanism by which elevated **D** produces this anemia pattern?

Explanation

## Why option 1 is right The clinical anchor is that hepcidin (structure **D**) is the master regulator of iron homeostasis secreted by the liver. In inflammation (via IL-6 → STAT3 signaling), hepcidin is upregulated. Elevated hepcidin binds to ferroportin (the basolateral iron exporter on duodenal enterocytes and macrophages), causing its internalization and degradation. This blocks iron release into the circulation and sequesters iron within macrophages and enterocytes. The result is the classic anemia of chronic disease/inflammation pattern: low serum iron, low transferrin saturation, but elevated ferritin (iron trapped in tissue stores). This is "functional iron deficiency" — iron is present but unavailable for erythropoiesis. (Harper 32e Ch 58; Harrison 21e Ch 95) ## Why each distractor is wrong - **Option 2**: Hepcidin does not competitively inhibit transferrin binding. Transferrin (structure **C**) is a plasma transport protein; hepcidin's action is on ferroportin, not on transferrin-iron interactions. This confuses the mechanism of iron transport with the mechanism of iron absorption regulation. - **Option 3**: Hepcidin does not directly suppress erythropoietin (EPO). While anemia of chronic disease involves relative EPO insufficiency, this is not the primary mechanism of hepcidin action. Hepcidin's effect is on iron availability, not EPO synthesis. This distractor conflates two separate pathways in anemia of chronic disease. - **Option 4**: Elevated hepcidin *decreases* intestinal iron absorption by degrading ferroportin (structure **B**), not by increasing DMT1 (structure **A**). This option reverses the physiologic effect and would result in iron deficiency, not iron sequestration with elevated ferritin. **High-Yield:** Anemia of chronic disease = elevated hepcidin → ferroportin degradation → iron sequestration in macrophages → low serum iron + low transferrin saturation + HIGH ferritin (distinguishes from iron deficiency anemia where ferritin is LOW). [cite: Harper 32e Ch 58; Harrison 21e Ch 95]