**Core Concept**
A leukemoid reaction and chronic myeloid leukemia (CML) are two distinct conditions that present with elevated white blood cell counts. Differentiating between these conditions is crucial for appropriate management and treatment. The underlying principle being tested here involves understanding the laboratory characteristics that distinguish these two entities.

**Why the Correct Answer is Right**
The correct answer is **B. Presence of the Philadelphia chromosome**. This is because the Philadelphia chromosome, resulting from a reciprocal translocation between chromosomes 9 and 22 (t(9;22)), is a hallmark of CML. In contrast, leukemoid reactions are reactive processes that do not involve chromosomal abnormalities. The presence of the Philadelphia chromosome in a patient with a leukocytosis suggests CML, whereas its absence supports a diagnosis of a leukemoid reaction. The Philadelphia chromosome is formed due to the fusion of the BCR and ABL genes, leading to the production of a constitutively active tyrosine kinase that promotes cell proliferation and survival.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a normal white blood cell count does not differentiate between CML and a leukemoid reaction. Both conditions can present with elevated white blood cell counts.
* **Option C:** This option is incorrect because the presence of immature granulocytes, known as blasts, is not specific for CML. Blasts can be present in both CML and leukemoid reactions, although the proportion of blasts is typically higher in CML.

**Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of the Philadelphia chromosome is highly specific for CML, but its absence does not rule out the diagnosis. Further diagnostic testing, such as molecular studies or bone marrow examination, may be necessary to confirm the diagnosis.

**Correct Answer:** B. Presence of the Philadelphia chromosome.