**Core Concept**
The size of splenomegaly in chronic myeloid leukemia (CML) is a prognostic indicator, reflecting the extent of bone marrow infiltration and extramedullary disease. In CML, the spleen becomes enlarged due to the accumulation of leukemic cells, which is often correlated with the severity of the disease.

**Why the Correct Answer is Right**
In CML, the size of splenomegaly is a significant prognostic factor. A larger spleen size indicates a more extensive involvement of the bone marrow and extramedullary organs, which correlates with a poorer prognosis. This is because a larger spleen size suggests a higher leukemic cell burden, increased risk of complications, and potential resistance to therapy. The spleen size can be used as a surrogate marker to assess the severity of the disease and guide treatment decisions.

**Why Each Wrong Option is Incorrect**
**Option B:** Phagocytic activity of WBC is reduced - This statement is incorrect because, in CML, the phagocytic activity of white blood cells (WBCs) is actually increased, leading to a phenomenon known as "leukocyte alkaline phosphatase" (LAP) score elevation.

**Option C:** Sudan black stain is specific for myeloblast - This statement is incorrect because Sudan black stain is a nonspecific stain that can stain various types of cells, including myeloblasts, but also other cells like histiocytes and macrophages.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that the size of splenomegaly in CML can be used to estimate the risk of transformation to blast crisis, a life-threatening complication of the disease.

**Correct Answer:**
✓ Correct Answer: A. Size of splenomegaly indicates prognosis