## **Core Concept**
The development of red blood cells (RBCs), or erythropoiesis, involves a series of steps from hematopoietic stem cells to mature erythrocytes. A key event in this process is the synthesis of hemoglobin, which is crucial for oxygen transport. The appearance of hemoglobin marks a significant stage in RBC maturation.

## **Why the Correct Answer is Right**
Hemoglobin first appears in the **basophilic erythroblast (or early normoblast)** stage of RBC development. During this stage, the cell begins to express specific genes that lead to the production of hemoglobin. This stage follows the pronormoblast (or proerythroblast) stage and precedes the polychromatophilic erythroblast stage. The presence of hemoglobin starts to change the staining properties of the cell, making it identifiable under a microscope.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the pronormoblast (or proerythroblast) stage precedes the appearance of hemoglobin. At this stage, the cell has large nuclei and a small amount of cytoplasm but has not yet started to produce hemoglobin.
- **Option B:** This option suggests a later stage than correct. By the polychromatophilic erythroblast stage, the cell has already started producing hemoglobin, and its cytoplasm shows a mix of basophilic and acidophilic staining due to the presence of ribosomal RNA and hemoglobin, respectively.
- **Option D:** This option refers to a very late stage in RBC development. Reticulocytes are immature RBCs that have extruded their nucleus and still contain remnants of ribosomal RNA, which can be visualized with special stains. They have already accumulated significant amounts of hemoglobin.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the process of erythropoiesis is tightly regulated and involves the coordinated action of various growth factors, including erythropoietin. Understanding the stages of RBC development, including when hemoglobin first appears, is essential for diagnosing and managing disorders of erythropoiesis, such as anemia.

## **Correct Answer:** . Basophilic erythroblast.