**Core Concept**
Blood-stained nipple discharge is a symptom that can be associated with various underlying conditions, primarily related to the breast or the ductal system. It is essential to differentiate between benign and malignant causes, as the management and prognosis vary significantly.

**Why the Correct Answer is Right**
The correct answer is linked to the presence of a tumor or cyst within the breast tissue, causing the disruption of normal ductal function and leading to the presence of blood in the nipple discharge. This is often due to the erosion of a tumor into the ductal system, resulting in the release of blood into the nipple. The most common tumors associated with this symptom are intraductal papillomas or breast carcinomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Galactorrhea, a condition characterized by the spontaneous flow of milk from the nipple, is not typically associated with blood-stained discharge.

**Option B:** Mastitis, an inflammation of the breast tissue, usually presents with signs of infection such as redness, swelling, and pain, rather than blood-stained discharge.

**Option C:** Fibrocystic changes, a common condition affecting the breast tissue, can cause cyclic breast pain and modularity but are not typically associated with blood-stained nipple discharge.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that blood-stained nipple discharge in a woman over 40 years old should prompt further investigation, including imaging studies and potentially a biopsy, to rule out breast malignancy.

**Correct Answer: D. Paget's disease of the breast.**