**Core Concept**
In the context of lymphoma, extranodal sites refer to locations outside the lymphatic system where cancer cells can spread and form tumors. The 14th World Conference on Lymphoma (14W) emphasizes the importance of identifying these sites to improve diagnosis and treatment outcomes.

**Why the Correct Answer is Right**
The central nervous system (CNS) is the most common extranodal site of lymphoma in immunocompromised patients, particularly those with HIV/AIDS. This is due to the ability of lymphoma cells to disseminate through the bloodstream and cross the blood-brain barrier. In the CNS, lymphoma cells can infiltrate the brain, spinal cord, and meninges, leading to severe neurological symptoms and complications. The involvement of the CNS in lymphoma is often associated with a poor prognosis and requires aggressive treatment.

**Why Each Wrong Option is Incorrect**
**Option B:** Gastrointestinal (GIT) lymphomas are common, but they predominantly occur in the small intestine, followed by the stomach and large intestine. While GIT lymphomas can be aggressive, they are not the most common extranodal site in the context of 14W.

**Option C:** Retroperitoneal lymph node involvement is a common site for lymphoma, but it is not the most common extranodal site. Retroperitoneal lymphadenopathy can occur in various types of lymphoma, but it is not as frequently associated with CNS involvement as other sites.

**Option D:** Mediastinal lymphoma is a significant concern, especially in the context of Hodgkin lymphoma. However, it is not the most common extranodal site of lymphoma, and its involvement is more commonly associated with respiratory and cardiovascular complications rather than CNS symptoms.

**Clinical Pearl / High-Yield Fact**
The involvement of the CNS in lymphoma is a medical emergency that requires prompt recognition and treatment. A high index of suspicion for CNS lymphoma should be maintained in patients with severe neurological symptoms, particularly those with a history of immunosuppression or previous lymphoma.

**✓ Correct Answer: A. CNS**