## **Core Concept**
Erythropoiesis is the process by which red blood cells (erythrocytes) are produced. It involves several stages, including **proerythroblast**, **basophilic erythroblast**, **polychromatophilic erythroblast**, and **orthochromic erythroblast**, before finally maturing into a reticulocyte and then an erythrocyte. Hemoglobin synthesis is a critical aspect of this process.

## **Why the Correct Answer is Right**
Hemoglobin starts to appear during the **basophilic erythroblast** stage of erythropoiesis. At this stage, the cell begins to accumulate **ribosomal RNA** and starts the synthesis of **hemoglobin**. The presence of hemoglobin is first detectable in the **polychromatophilic erythroblast** due to the accumulation of hemoglobin, but its synthesis begins in the previous stage.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This stage represents the earliest form of red blood cell production and is primarily involved in cell division and the beginning of erythropoietic differentiation. It does not yet show significant hemoglobin synthesis.
- **Option B:** While this stage does show some initial signs of differentiation towards red blood cell lineage, significant hemoglobin accumulation is not a characteristic feature here; it's more about the cell becoming committed to the erythroid lineage.
- **Option D:** By this stage, the cell has almost completed its maturation and has a high concentration of hemoglobin, but it is not the stage when hemoglobin first appears.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **hemoglobin synthesis** starts early in erythropoiesis but becomes more pronounced and visible under microscopy during the **polychromatophilic stage**. This is a critical aspect of understanding red blood cell production and maturation.

## **Correct Answer:** .