**Core Concept**
Hepcidin is a peptide hormone that regulates iron metabolism by controlling iron absorption and recycling. It plays a crucial role in maintaining iron homeostasis by binding to the iron exporter ferroportin, leading to its internalization and degradation. This action effectively reduces the amount of iron available for absorption and utilization by the body.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism by which hepcidin exerts its effects on iron metabolism. Hepcidin binds to ferroportin, a transmembrane protein responsible for exporting iron from cells, thereby preventing iron from entering the bloodstream. This leads to a decrease in serum iron levels and a reduction in the availability of iron for various bodily functions. The downregulation of ferroportin is a key step in hepcidin's regulation of iron metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hepcidin does not regulate the expression of transferrin receptors, which are involved in the uptake of iron from the bloodstream.

**Option B:** This option is incorrect because hepcidin does not directly affect the expression of ferritin, a protein that stores iron in cells.

**Option C:** This option is incorrect because hepcidin does not regulate the expression of the heme oxygenase enzyme, which is involved in the breakdown of heme to release iron.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hepcidin regulation is crucial in conditions such as iron overload (hemochromatosis) and iron deficiency anemia. Understanding the role of hepcidin in iron metabolism can help clinicians diagnose and manage these conditions more effectively.

**Correct Answer: C. Ferroportin**