**Core Concept**
Paget's disease of the nipple is a rare form of breast cancer characterized by the presence of malignant cells in the epidermis of the nipple and, occasionally, the areola. It is often associated with an underlying ductal carcinoma in situ (DCIS) or invasive breast cancer.

**Why the Correct Answer is Right**
Paget's disease is typically treated with mastectomy, which involves the surgical removal of the breast. This is because the disease often involves the nipple-areolar complex and may not be completely excised with a simple lumpectomy. In some cases, a skin-sparing mastectomy may be performed to preserve the skin over the chest, followed by breast reconstruction. The goal of treatment is to remove all cancerous cells and prevent recurrence.

**Why Each Wrong Option is Incorrect**
**Option A:** Lumpectomy is often not sufficient for Paget's disease due to its extensive involvement of the nipple-areolar complex.
**Option B:** Wide local excision may not be adequate to remove the underlying DCIS or invasive cancer, making mastectomy a more appropriate choice.
**Option C:** Radiation therapy may be used as an adjunct to surgery in some cases, but it is not a primary treatment for Paget's disease.

**Clinical Pearl / High-Yield Fact**
Paget's disease of the nipple often presents with a characteristic eczematous rash in the nipple, which may be misdiagnosed as a benign skin condition. A biopsy should be performed to confirm the diagnosis.

**Correct Answer:** C. Mastectomy.