## **Core Concept**
Post-transplant lymphoproliferative disorders (PTLD) are a well-recognized complication of solid organ transplantation and hematopoietic stem cell transplantation. These disorders arise due to the immunosuppressed state of the patient, which allows for the uncontrolled proliferation of lymphocytes.

## **Why the Correct Answer is Right**
The correct answer, **B. B cells**, is right because PTLD is primarily caused by the proliferation of B cells. This proliferation is often driven by Epstein-Barr virus (EBV), which infects B cells and can lead to their malignant transformation, especially in the setting of immunosuppression. EBV-negative PTLD can also occur, but B cells are the predominant cell type involved.

## **Why Each Wrong Option is Incorrect**
- **Option A:** T cells are not the primary cells involved in PTLD. While T-cell lymphomas can occur post-transplant, they are less commonly associated with PTLD than B-cell lymphoproliferations.
- **Option C:** Plasma cells are mature B cells that produce antibodies. While they are derived from B cells, PTLD is specifically related to the proliferation of B cells at various stages of differentiation, not just plasma cells.
- **Option D:** Natural killer (NK) cells are a type of lymphocyte that can be involved in lymphoproliferative disorders, but they are not the primary cell type associated with PTLD.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that PTLD often presents with fever, night sweats, weight loss, and lymphadenopathy, similar to lymphoma in immunocompetent patients. The risk of PTLD increases with the level and duration of immunosuppression. Reduction of immunosuppression is a common initial approach to managing PTLD, along with specific treatments like rituximab for EBV-positive B-cell PTLD.

## **Correct Answer:** B. B cells