## **Core Concept**
Human hematopoietic stem cells (HSCs) are characterized by their ability to self-renew and differentiate into all blood cell types. Surface glycoproteins play a crucial role in identifying and isolating these cells. The most commonly used markers for human HSCs include CD34, CD38, CD90, and others.

## **Why the Correct Answer is Right**
The correct answer, **CD34**, is a surface glycoprotein that is widely used as a marker for hematopoietic stem cells and progenitor cells. CD34 is expressed on the surface of early lymphoid and myeloid cells, endothelial progenitor cells, and hematopoietic stem cells. It is often used in clinical and research settings to identify, quantify, and isolate HSCs for bone marrow transplantation and other therapeutic applications. The expression of CD34 helps in distinguishing HSCs from more differentiated cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While some surface markers are specific to certain lineages or stages of hematopoiesis, without the specific option provided, we acknowledge that not all surface glycoproteins are equally expressed on HSCs.
- **Option B:** Similarly, other surface markers might be expressed on HSCs but are not as universally recognized or specific for these cells as CD34.
- **Option D:** This option, like the others, would represent surface glycoproteins that are either not as specifically associated with HSCs or are not the most commonly used marker for these cells.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **CD34+ cell count** is a critical measure for the success of hematopoietic stem cell transplantation. The number of CD34+ cells infused can predict engraftment and long-term outcomes. Thus, understanding the role of CD34 in HSC identification is crucial for both clinical practice and research.

## **Correct Answer:** . CD34.