**Core Concept**
Target cells are a type of abnormal red blood cell seen in conditions characterized by a combination of increased 2,3-bisphosphoglycerate (2,3-BPG) levels and decreased hemoglobin concentration, leading to a characteristic "bull's eye" appearance under the microscope.

**Why the Correct Answer is Right**
The increased 2,3-BPG levels in target cells bind to hemoglobin, reducing its affinity for oxygen and increasing the release of oxygen to tissues. This adaptation helps to compensate for the decreased hemoglobin concentration, which can occur due to chronic diseases such as hemoglobinopathies, thalassemias, or chronic hypoxia. The decreased hemoglobin concentration and increased 2,3-BPG levels lead to a characteristic change in the shape of red blood cells, resulting in the formation of target cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Target cells are not typically seen in conditions characterized by increased red blood cell size, such as polycythemia vera or erythrocytosis.
**Option B:** Target cells are not typically associated with conditions characterized by increased red blood cell membrane fragility, such as hereditary spherocytosis.
**Option C:** Target cells are not typically seen in conditions characterized by decreased red blood cell membrane surface area, such as hereditary elliptocytosis.

**Clinical Pearl / High-Yield Fact**
Target cells are a key diagnostic feature in conditions such as beta-thalassemia major, where they can be seen in up to 50% of patients. The presence of target cells should prompt further investigation into the underlying cause of the anemia.

**Correct Answer: D**