## **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 expressed surface glycoprotein in human HSCs is CD34.

## **Why the Correct Answer is Right**
CD34 is a transmembrane phosphoglycoprotein expressed on early lymphoid and myeloid cells, endothelial progenitor cells, and hematopoietic progenitor cells, including HSCs. It is widely used as a marker for isolating HSCs for bone marrow transplantation and research. The mechanism behind CD34's role involves its participation in cell adhesion, possibly facilitating the migration and homing of HSCs to the bone marrow.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While some surface markers can be expressed on HSCs, without specific details, it's hard to directly refute. However, given the context, CD34 is more specifically associated with HSCs than other markers.
- **Option B:** Similarly, without specifics, it's challenging to address directly, but CD34 is a well-established marker.
- **Option C:** This option might represent another surface marker, but CD34 is notably the most commonly referenced for human HSCs.
- **Option D:** This could represent a marker for a more differentiated cell type or a marker not specifically associated with HSCs.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **CD34+ cells** are often used in the context of **bone marrow transplantation**. The number of CD34+ cells infused can predict the rate of engraftment and the risk of graft-versus-host disease. Thus, understanding CD34's role is crucial for hematologists and oncologists.

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