**Core Concept**
Iron deficiency anemia is a common condition that requires oral iron supplementation to replenish iron stores. The choice of iron formulation is crucial in managing adverse effects and improving patient compliance.

**Why the Correct Answer is Right**
Gastrointestinal side effects are a major cause of poor compliance with oral iron therapy. The most common adverse effect is **gastrointestinal upset**, including nausea, vomiting, abdominal pain, and diarrhea. These symptoms are primarily due to the **gastric acid stimulation** caused by iron salts, leading to **mucosal irritation** and **inflammation** in the stomach and small intestine. The iron salts commonly used in oral iron therapy, such as ferrous sulfate and ferrous fumarate, are poorly tolerated by many patients, leading to a high dropout rate.

**Why Each Wrong Option is Incorrect**
* **Option A:** Constipation is not a common adverse effect of oral iron therapy.
* **Option B:** Allergic reactions are rare and not a primary reason for poor compliance.
* **Option C:** Hemochromatosis is a genetic disorder that leads to iron overload, not a consequence of oral iron therapy.

**Clinical Pearl / High-Yield Fact**
To minimize gastrointestinal side effects and improve patient compliance, it is essential to start with a low dose of iron and gradually increase it to the recommended dose. Additionally, taking iron supplements with food, especially meals rich in vitamin C, can help reduce gastrointestinal upset.

**Correct Answer:** C.