**Core Concept**
Iron deficiency anemia is a common condition characterized by a lack of sufficient iron to produce hemoglobin, leading to decreased oxygen delivery to tissues. Treatment typically involves replenishing iron stores with oral supplements, which can have distinct effects on the body.

**Why the Correct Answer is Right**
When taking oral iron supplements, patients often experience gastrointestinal side effects, such as nausea, vomiting, and abdominal pain, due to the direct contact of iron with the gastric mucosa. These symptoms are usually dose-dependent and can be minimized by taking iron supplements with food, particularly those high in vitamin C, which enhances iron absorption. The characteristic "black stool" or "black tongue" is another notable feature, resulting from the oxidation of iron to its ferric state.

**Why Each Wrong Option is Incorrect**
**Option A:** While gastrointestinal side effects are common, they are not the only characteristic features of oral iron supplementation. Option A is a correct statement, but not the exception.
**Option B:** This option is not a characteristic feature of oral iron supplementation. In fact, taking iron supplements with food can help mitigate gastrointestinal side effects.
**Option C:** Black stool or tongue is indeed a characteristic feature of oral iron supplementation, resulting from the oxidation of iron to its ferric state.

**Clinical Pearl / High-Yield Fact**
When prescribing oral iron supplements, it's essential to instruct patients to take them with food, particularly those high in vitamin C, to enhance absorption and reduce gastrointestinal side effects.

**Correct Answer: B. This option is not a characteristic feature of oral iron supplementation.**

✓ Correct Answer: D. The treatment should be discontinued immediately once hemoglobin normalizes to prevent side effects of iron.