Okay, let's tackle this question. The question is about which surface glycoprotein is most often expressed in human hematopoietic stem cells. Hmm, I remember that stem cells have specific markers. CD34 comes to mind. Wait, CD34 is a well-known marker for hematopoietic stem cells. But wait, there are other CD markers too. Let me think.

Hematopoietic stem cells are found in the bone marrow and are responsible for producing all blood cells. Their surface markers are important for identification. CD34 is definitely one of them. Also, maybe CD117 (c-KIT) is another marker. But the question is about the most often expressed. CD34 is more commonly associated with these stem cells.

Wait, CD34 is a cell surface glycoprotein that's used to identify these cells. The other options could be things like CD45, which is a marker for leukocytes but not specifically stem cells. Or CD44, which might be present on other cells too. So the key here is to remember that CD34 is the primary marker for hematopoietic stem cells.

Let me double-check. Yes, CD34 is the most commonly used marker for identifying and isolating hematopoietic stem cells. Other markers like CD133 or CD90 might be present but aren't as specific. So the correct answer should be CD34.

**Core Concept**
Human hematopoietic stem cells (HSCs) are identified by specific cell surface glycoproteins. **CD34** is the hallmark antigen for these cells, distinguishing them from mature blood cells and other progenitor populations.

**Why the Correct Answer is Right**
**CD34** is a transmembrane glycoprotein expressed on the surface of undifferentiated HSCs. It is used in clinical and research settings to isolate HSCs via flow cytometry or magnetic bead separation. While CD34 expression diminishes as HSCs differentiate, it remains the most specific and widely used marker for these cells.

**Why Each Wrong Option is Incorrect**
**Option A:** **CD45** is a leukocyte common antigen expressed on all nucleated hematopoietic cells, not specific to stem cells.
**Option B:** **CD44** is a cell adhesion molecule found on various cell types, including stem cells, but lacks specificity for hematopoietic stem cells.
**Option C:** **CD117 (c-KIT)** is expressed on HSCs but also on mast cells and other progenitors, making it less specific than CD34.

**Clinical Pearl / High-Yield Fact**
**CD34+ cells** are the gold standard for isolating HSCs in bone marrow transplants. Remember: *CD34 is the "stem cell label"*—absent on mature cells, critical for regenerative medicine.

**Correct Answer: D. CD34**

✓ Correct Answer: D. CD34