**Core Concept**
The myeloid series is a group of cells that develop from the hematopoietic stem cell and give rise to various types of blood cells, including neutrophils, eosinophils, basophils, monocytes, and erythrocytes. The CD markers are proteins expressed on the surface of these cells, which can be used as specific markers to identify different cell types.

**Why the Correct Answer is Right**
CD117, also known as c-Kit, is a transmembrane receptor tyrosine kinase that is specifically expressed on the surface of hematopoietic stem cells and early myeloid cells. It plays a crucial role in the proliferation, differentiation, and survival of myeloid cells. The expression of CD117 is lost as the cells mature into different types of myeloid cells. Therefore, CD117 is a useful marker for identifying the myeloid lineage, particularly in the context of acute myeloid leukemia (AML).

**Why Each Wrong Option is Incorrect**
**Option A:** CD34 is a marker for hematopoietic stem cells and progenitor cells, but it is not specific for the myeloid series, as it is also expressed on other types of hematopoietic cells, including lymphoid cells.

**Option B:** CD45 is a pan-leukocyte marker that is expressed on the surface of all leukocytes, including both myeloid and lymphoid cells. It is not specific for the myeloid series.

**Option C:** CD99 is a marker for Ewing's sarcoma and some types of leukemia, but it is not specific for the myeloid series.

**Clinical Pearl / High-Yield Fact**
CD117 is often used as a diagnostic marker for acute myeloid leukemia (AML), particularly in the context of the FAB (French-American-British) classification system. AML with maturation is characterized by the presence of CD117-positive cells in the bone marrow.

**✓ Correct Answer: D. CD117**