**Core Concept**
Richter transformation, also known as Richter syndrome, is a rare and severe complication of chronic lymphocytic leukemia (CLL). It involves the transformation of CLL into a more aggressive and diffuse large B-cell lymphoma (DLBCL), which is a type of non-Hodgkin lymphoma.

**Why the Correct Answer is Right**
The transformation of CLL to DLBCL is thought to occur due to the acquisition of genetic mutations that disrupt the normal function of the B-cell receptor signaling pathway. This leads to the uncontrolled proliferation of malignant B cells, which is characteristic of DLBCL. The transformation is often associated with the presence of MYC rearrangements, which are a common feature of DLBCL.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the transformation of CLL.
* **Option B:** This option is incorrect because CLL does not typically transform into follicular lymphoma.
* **Option C:** This option is incorrect because CLL does not typically transform into mantle cell lymphoma.

**Clinical Pearl / High-Yield Fact**
The transformation of CLL to DLBCL is a medical emergency that requires prompt treatment with chemotherapy and/or targeted therapy. It is essential to recognize the signs and symptoms of transformation, which can include rapid disease progression, fever, and weight loss.

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