**Core Concept**
Non-Hodgkin's lymphoma (NHL) is a type of cancer that arises from the lymphatic system, with the potential to spread to various sites outside the lymph nodes. The extranodal sites refer to organs or tissues outside the lymphatic system where NHL can metastasize. In clinical practice, identifying the most common extranodal site for NHL is crucial for diagnosis and treatment planning.

**Why the Correct Answer is Right**
The correct answer is the central nervous system (CNS), particularly the brain. This is due to the propensity of certain NHL subtypes, such as diffuse large B-cell lymphoma (DLBCL), to infiltrate the CNS. The pathophysiology involves the malignant lymphoma cells spreading through the bloodstream or cerebrospinal fluid to reach the brain. The presence of lymphoma cells in the CNS can lead to neurological symptoms, seizures, or even increased intracranial pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Bone marrow is a common site for NHL, but it is not the most common extranodal site. NHL can infiltrate the bone marrow, leading to anemia, thrombocytopenia, or leukopenia.
* **Option B:** Liver is an organ that can be involved in NHL, particularly in the context of systemic disease or as a secondary site. However, it is not the most common extranodal site.
* **Option C:** Lung is another organ that can be involved in NHL, especially in the context of secondary spread or as part of a systemic disease. However, it is not the most common extranodal site.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with NHL, it is essential to consider the possibility of CNS involvement, especially in patients with DLBCL or other high-grade lymphomas. A lumbar puncture (LP) may be necessary to assess for CNS involvement, which can significantly impact treatment planning and prognosis.

**Correct Answer: C. CNS**