**Core Concept**
Axillary lymph nodes are the primary site of metastasis for breast cancer, with the first involved lymph node typically being the **pectoralis minor lymph node** or the **lateral group of axillary lymph nodes**. This is due to the lymphatic drainage of the breast, which primarily follows the subdermal plexus along the lateral aspect of the breast.

**Why the Correct Answer is Right**
The pectoralis minor lymph node is the first echelon lymph node involved in breast cancer metastasis because it receives lymph from the lateral breast and the anterior chest wall. The lymphatic vessels from the breast then drain into the pectoral minor lymph node, which is a small node located beneath the pectoralis minor muscle in the axilla. From there, the cancer cells can spread to the lateral group of axillary lymph nodes, which are the next echelon of lymph nodes involved in the disease process.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified, so it cannot be evaluated.
* **Option B:** This option is incorrect because the internal mammary lymph nodes are involved in breast cancer metastasis, but they are not the first involved lymph nodes.
* **Option C:** This option is incorrect because the sentinel lymph node, while a key concept in breast cancer, is not a specific lymph node but rather the first lymph node to receive drainage from the primary tumor site.

**Clinical Pearl / High-Yield Fact**
The pattern of lymph node involvement in breast cancer can be remembered using the "3-4-5" rule, where the 3-4-5 refers to the number of lymph nodes involved in each echelon: 3-4 lymph nodes in the pectoral minor group, 4-5 lymph nodes in the lateral group, and 5 or more lymph nodes in the medial group.

**Correct Answer:** C. Lateral group of axillary lymph nodes.