**Core Concept**
Chest wall involvement in breast cancer refers to the infiltration or invasion of tumor cells into the structures that make up the chest wall. This includes the ribs, sternum, intercostal muscles, and the endothoracic fascia.

**Why the Correct Answer is Right**
The correct answer involves understanding the anatomical boundaries of the chest wall. The endothoracic fascia is a layer of connective tissue that surrounds the pleura and attaches it to the anterior aspect of the thoracic cage. Chest wall involvement in breast cancer typically means tumor invasion into the ribs, sternum, intercostal muscles, or the endothoracic fascia. However, the pectoralis major muscle is not considered part of the chest wall in this context.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the ribs are indeed part of the chest wall.
**Option B:** Incorrect because the sternum is also part of the chest wall.
**Option C:** Incorrect because the intercostal muscles are a component of the chest wall.
**Option D:** Incorrect because the endothoracic fascia is a layer of connective tissue that surrounds the pleura and attaches it to the anterior aspect of the thoracic cage.

**Clinical Pearl / High-Yield Fact**
In breast cancer staging, chest wall involvement is an important prognostic factor and can significantly affect treatment plans. It is essential for clinicians to accurately assess the extent of chest wall involvement to determine the most appropriate course of treatment.

**Correct Answer: D. Pectoralis major muscle.**