## **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 commonly used in clinical and research settings to identify, quantify, and isolate HSCs for bone marrow transplantation and other therapeutic applications.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While some stem cells and progenitor cells express **CD90 (Thy-1)**, it is not as universally recognized or specific for human hematopoietic stem cells as CD34.
- **Option B:** **CD38** is a marker that can be found on many hematopoietic cells, including some stem cells, but it is not as specific for hematopoietic stem cells as CD34 and can also be expressed on activated T cells and other cell types.
- **Option D:** Without a specified option, we focus on the provided correct answer.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **CD34+ cells** are critical for hematopoietic reconstitution following bone marrow transplantation. The number of CD34+ cells infused can predict the rate of engraftment and long-term hematopoietic function. Therefore, assessing CD34 expression is a vital step in the preparation and evaluation of bone marrow or stem cell transplants.

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