## **Core Concept**
The cortex of a lymph node is the outer layer where lymphatic nodules and germinal centers are located, playing a crucial role in the immune response. It is rich in lymphocytes and contains structures essential for immune cell activation and proliferation.

## **Why the Correct Answer is Right**
The correct answer, **C. Germinal centers**, is right because germinal centers are found within the cortex of lymph nodes. These are sites where B lymphocytes proliferate, differentiate, and mutate to become antibody-secreting plasma cells or memory B cells. Germinal centers are critical for the development of immune responses, especially for the production of high-affinity antibodies.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, while the lymph node does contain a medulla, the question specifically asks about the cortex.
- **Option B:** This option is incorrect as it does not specify a structure known to be associated with the lymph node cortex.
- **Option D:** This option is incorrect because, although lymphatic nodules are indeed found in the cortex of lymph nodes and are involved in the immune response, they are not as specifically characteristic of the cortex as germinal centers.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that germinal centers within lymph node cortices are sites of **somatic hypermutation** and **class switch recombination**, critical processes for generating high-affinity antibodies. Abnormalities in germinal center formation or function can lead to immunodeficiency or autoimmune diseases.

## **Correct Answer:** C. Germinal centers.