**Core Concept**
Richter's transformation refers to the development of **diffuse large B-cell lymphoma** in patients with pre-existing **chronic lymphocytic leukemia (CLL)**. This transformation is a significant event in the natural history of CLL, often associated with a poorer prognosis. The underlying mechanism involves **clonal evolution** and **genetic mutations**.

**Why the Correct Answer is Right**
The correct answer is associated with Richter's transformation due to the **aggressive nature** of the transformed lymphoma, which often presents with **rapid progression** and **new onset of symptoms**. The transformation is thought to occur due to **acquisition of new genetic mutations** in the CLL clone, leading to a more **aggressive phenotype**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect due to lack of direct association with Richter's transformation.
**Option B:** Incorrect as it is not a recognized complication of CLL.
**Option C:** Incorrect due to its association with a different clinical entity.

**Clinical Pearl / High-Yield Fact**
Richter's transformation is a critical diagnosis to consider in patients with CLL who experience **sudden worsening** of their condition, as it requires **prompt initiation of aggressive therapy**.

**Correct Answer:** D. Diffuse large B-cell lymphoma.