**Core Concept**
The question is testing the concept of gastrointestinal bleeding and its clinical manifestations. Specifically, it is asking about the amount of blood loss required to produce a single black stool, also known as melena. Melena is a sign of upper gastrointestinal bleeding, usually from the upper small intestine or the stomach.

**Why the Correct Answer is Right**
The amount of blood loss required to produce melena is approximately 50-100 mL. This is because the blood is exposed to the acidic environment of the stomach and the digestive enzymes in the small intestine, which lead to the breakdown of the blood into a dark, tarry substance. The presence of this substance in the stool is a sign of upper gastrointestinal bleeding. The blood loss is usually slow and chronic, which is why the amount of blood required to produce melena is relatively small.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide a specific amount of blood loss. While it is true that the amount of blood loss required to produce melena can vary, a specific amount is required to answer the question.
* **Option B:** This option is incorrect because the amount of blood loss required to produce melena is not as high as 200 mL. This amount of blood loss would likely produce more severe symptoms and signs of bleeding.
* **Option C:** This option is incorrect because it is not a specific amount of blood loss. While it is true that the amount of blood loss required to produce melena can vary, a specific amount is required to answer the question.
* **Option D:** This option is incorrect because it is lower than the actual amount of blood loss required to produce melena. While some blood loss may be required to produce melena, 20 mL is not a sufficient amount.

**Clinical Pearl / High-Yield Fact**
It's worth noting that melena can be a sign of a serious underlying condition, such as a bleeding ulcer or a tumor in the upper gastrointestinal tract. Therefore, it is essential to investigate the cause of melena thoroughly and to consider the possibility of a serious underlying condition.

**Correct Answer:** D. 20 mL