**Core Concept**
Fibroadenoma is a common benign breast tumor, typically presenting as a painless, mobile, and well-defined lump in young females. This condition is characterized by the proliferation of both epithelial and stromal elements within the breast tissue.

**Why the Correct Answer is Right**
Fibroadenomas arise from the terminal duct-lobular unit of the breast and are composed of a mixture of glandular and fibrous tissue. They are often found in women under the age of 30 and are more common in African-American women. The growth of fibroadenomas is influenced by hormonal fluctuations, particularly estrogen, which promotes the proliferation of epithelial cells. Histologically, fibroadenomas are distinguished by their characteristic "cutaneous" or "leaf-like" appearance, with a mixture of glandular and fibrous elements.

**Why Each Wrong Option is Incorrect**
**Option B:** Fibroadenosis, also known as fibrocystic disease, is a condition characterized by the presence of multiple cysts and fibrous tissue within the breast. While it can present with breast lumps, it is typically associated with pain and tenderness, particularly in the premenstrual phase.

**Option C:** Breast cancer is a malignant tumor that can present as a painless lump, but it is often associated with other risk factors, such as a family history of breast cancer. In contrast, fibroadenomas are typically benign and have a distinct histological appearance.

**Option D:** Mastalgia refers to breast pain, which can be a symptom of various conditions, including fibroadenosis or breast cancer. However, it is not a diagnosis in itself and does not describe a specific type of breast lump.

**Clinical Pearl / High-Yield Fact**
Fibroadenomas are typically benign and do not require surgical removal unless they are large, symptomatic, or cause cosmetic concern. A clinical pearl to remember is that fibroadenomas are often "mobile" and "well-defined," whereas malignant tumors tend to be "fixed" and "irregular."

**✓ Correct Answer: A. Fibroadenoma**