**Core Concept**
Fibroadenosis, also known as fibrocystic breast disease, is a benign condition characterized by the presence of fibrous and glandular tissue in the breast, leading to symptoms such as breast pain, lumpiness, and tenderness. This condition is influenced by hormonal fluctuations, particularly estrogen, which causes the growth and proliferation of breast tissue.

**Why the Correct Answer is Right**
The most common presenting symptom of fibroadenosis is breast pain, often described as cyclic or premenstrual. This pain is thought to be due to the proliferation of fibrous and glandular tissue in response to estrogen levels, which peaks just before menstruation. The exact mechanism involves the contraction of smooth muscle cells in the breast, leading to increased blood flow and inflammation, which exacerbates the pain.

**Why Each Wrong Option is Incorrect**
**Option A:** Breast lump is a symptom of fibroadenosis, but it is not the most common presenting symptom. Lumps are more characteristic of breast cancer or fibroadenoma.
**Option B:** Nipple discharge is not a typical symptom of fibroadenosis, and when it occurs, it is often associated with other conditions such as ductal ectasia or breast cancer.
**Option C:** Skin thickening is not a common symptom of fibroadenosis, although it can be seen in conditions such as sclerosing adenosis or phyllodes tumor.

**Clinical Pearl / High-Yield Fact**
Fibroadenosis is a benign condition, and most women with this diagnosis will not develop breast cancer. However, it is essential to evaluate any new breast symptoms, including pain, lumps, or nipple discharge, to rule out more serious conditions.

**Correct Answer: B. Nipple discharge**