**Core Concept**
Modified radical mastectomy (MRM) is a surgical procedure for breast cancer that involves the removal of the breast tissue, the underlying pectoralis major muscle is generally preserved, and the removal of some axillary lymph nodes to assess for metastasis. This procedure is a compromise between a simple mastectomy and a radical mastectomy, which involves the removal of the pectoralis major muscle.

**Why the Correct Answer is Right**
In a modified radical mastectomy, the surgeon removes the breast tissue, the ductal system, and some of the axillary lymph nodes to evaluate the extent of cancer spread. However, the underlying pectoralis major muscle is typically preserved, as its removal would significantly compromise the patient's arm function and cosmetic outcome. The pectoralis major muscle plays a crucial role in shoulder stability and arm movement, and its preservation is essential to maintain the patient's quality of life. The surgeon usually performs a partial axillary dissection, removing a selected number of lymph nodes, to assess for cancer spread.

**Why Each Wrong Option is Incorrect**
**Option B:** The pectoralis minor muscle is often removed during a modified radical mastectomy, as it is located between the pectoralis major and the axillary lymph nodes, and its removal provides better access to the axilla for lymph node dissection.
**Option C:** The axillary lymph nodes are partially removed during a modified radical mastectomy to evaluate the extent of cancer spread. This is a critical component of the procedure, as it helps determine the stage of cancer and guide further treatment.
**Option D:** The nipple is often removed during a modified radical mastectomy, especially if there is a high risk of cancer spread to the nipple-areolar complex. This decision is based on the tumor's location, size, and histological characteristics.

**Clinical Pearl / High-Yield Fact**
In breast cancer surgery, the extent of lymph node dissection and the preservation of surrounding tissues are critical factors in determining the patient's prognosis and quality of life. Surgeons must balance the need to remove cancerous tissue with the need to preserve essential structures and functions.

**✓ Correct Answer: A. Pectoralis major**