**Core Concept**
Post-transplant lymphoproliferative disorders (PTLDs) are a type of malignancy that arises in immunocompromised patients, particularly those who have received organ transplants. The underlying mechanism involves the reactivation of Epstein-Barr virus (EBV) and the subsequent proliferation of EBV-infected B lymphocytes. This condition is a major complication of solid organ transplantation and can occur in both adults and children.

**Why the Correct Answer is Right**
The correct answer is B. B cell because PTLDs are predominantly driven by the clonal expansion of EBV-infected B lymphocytes. EBV, a ubiquitous herpesvirus, infects over 90% of the world's population, and in immunocompetent individuals, the virus is typically controlled by the host's immune system. However, in immunocompromised patients, such as transplant recipients, EBV can reactivate and lead to the uncontrolled proliferation of EBV-infected B cells. This results in a variety of clinical manifestations, ranging from benign lymphoid hyperplasia to aggressive lymphomas.

**Why Each Wrong Option is Incorrect**
**Option A:** T cell lymphomas are less common in the context of post-transplant lymphoma and are not typically associated with EBV reactivation.

**Option C:** Null cell lymphomas are a rare type of T cell lymphoma that lacks specific T cell markers, but they are not typically associated with post-transplant lymphoproliferative disorders.

**Option D:** Natural Killer (NK) cell lymphomas are a type of rare and aggressive lymphoma, but they are not commonly associated with post-transplant lymphoma.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that PTLDs are a significant complication of solid organ transplantation, and their diagnosis often requires a high index of suspicion. A thorough understanding of the underlying pathogenesis, including EBV reactivation and B cell proliferation, is crucial for the proper management of these patients.

✓ Correct Answer: B. B cell