**Core Concept**
Chronic Lymphocytic Leukemia (CLL) is a type of cancer characterized by the clonal expansion of mature, malignant lymphocytes in the bone marrow and peripheral blood. CLL is the most common type of leukemia in adults, particularly affecting individuals over the age of 60.

**Why the Correct Answer is Right**
CLL is characterized by the accumulation of malignant B cells, which are arrested in the G0 phase of the cell cycle and are unresponsive to antigenic stimulation. This leads to an accumulation of these cells in the bone marrow and peripheral blood. The malignant cells in CLL typically express surface antigens such as CD5 and CD23, but lack surface immunoglobulin. The pathogenesis of CLL involves a complex interplay of genetic and epigenetic alterations, including mutations in the tumor suppressor gene TP53 and the oncogene ATM.

**Why Each Wrong Option is Incorrect**
**Option A:** CLL is characterized by the clonal expansion of mature, malignant T cells. **Incorrect** because CLL is primarily a B cell malignancy, not a T cell malignancy.
**Option B:** CLL is characterized by the accumulation of immature, blast-like cells. **Incorrect** because CLL is characterized by the accumulation of mature, lymphoid cells, not immature blast-like cells.
**Option C:** CLL is characterized by the overexpression of the BCL-2 protein. **Incorrect** because while BCL-2 overexpression does play a role in CLL, it is not a defining characteristic of the disease.

**Clinical Pearl / High-Yield Fact**
CLL is often associated with a variety of complications, including autoimmune cytopenias, lymphadenopathy, and splenomegaly. Patients with CLL should be monitored for these complications and treated accordingly.

**Correct Answer:** D.