**Core Concept**
Non-Hodgkin's lymphoma (NHL) is a type of cancer that affects the immune system, specifically the lymphatic system. The lymphatic system is a network of vessels, organs, and tissues that help defend the body against infection and disease. NHL can arise in various sites outside the lymph nodes.

**Why the Correct Answer is Right**
The most common site for NHL outside the lymph nodes is the **spleen**. The spleen is an organ that filters the blood and stores lymphocytes, a type of white blood cell. NHL can arise in the spleen due to the accumulation of lymphocytes, which can become malignant over time. This is often seen in patients with a history of lymphoma or those with a weakened immune system.

**Why Each Wrong Option is Incorrect**
* **Option A:** Bone marrow is a common site for NHL, but it is not the most common site outside the lymph nodes. Bone marrow is a spongy tissue inside the bones where blood cells are produced.
* **Option B:** The gastrointestinal tract is a possible site for NHL, but it is not the most common site outside the lymph nodes. NHL can arise in the stomach, small intestine, or other parts of the GI tract.
* **Option C:** The central nervous system (CNS) is a rare site for NHL, but it is not the most common site outside the lymph nodes. NHL can arise in the brain, spinal cord, or other parts of the CNS.

**Clinical Pearl / High-Yield Fact**
The spleen is a common site for NHL, especially in patients with a history of lymphoma or a weakened immune system. This is an important consideration for clinicians when evaluating patients with NHL, as it may require a different treatment approach.

**Correct Answer:** B.