**Core Concept**
Fulvestrant is a selective estrogen receptor degrader (SERD) that targets estrogen receptors (ER) in breast cancer cells, leading to the degradation of these receptors and ultimately inhibiting estrogen's proliferative effects on tumor cells.

**Why the Correct Answer is Right**
Fulvestrant works by binding to the estrogen receptor and inducing its degradation through the ubiquitin-proteasome pathway. This results in a decrease in estrogen receptor levels, which in turn leads to a decrease in the growth-promoting effects of estrogen on breast cancer cells. Fulvestrant is specifically used in the treatment of hormone receptor-positive metastatic breast cancer in postmenopausal women who have received prior endocrine therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** Fulvestrant is not primarily used as a chemotherapeutic agent; it is a targeted therapy that specifically targets estrogen receptors.
**Option B:** Fulvestrant is not a selective estrogen receptor modulator (SERM); it is a selective estrogen receptor degrader (SERD).
**Option C:** Fulvestrant is not used in the treatment of prostate cancer; it is used in the treatment of breast cancer.

**Clinical Pearl / High-Yield Fact**
Fulvestrant is administered as a monthly injection, and its efficacy is often monitored through serial assessments of tumor markers and imaging studies.

**Correct Answer:** C.