**Core Concept**
Cystosarcoma phylloides, also known as phyllodes tumor, is a rare breast tumor that can be benign, borderline, or malignant. The treatment of choice depends on the tumor's histological characteristics and the patient's overall health.

**Why the Correct Answer is Right**
The treatment of choice for cystosarcoma phylloides is surgical excision. For small, benign tumors, simple excision is often sufficient. However, for larger or malignant tumors, wide local excision with clear margins is recommended to prevent recurrence. In some cases, mastectomy may be necessary to achieve clear margins. The goal of surgery is to remove the tumor completely and prevent local recurrence.

**Why Each Wrong Option is Incorrect**
* **Option A:** Conservative management with observation or hormone therapy is not the treatment of choice for cystosarcoma phylloides, as these tumors are often malignant or have a high recurrence rate.
* **Option B:** Radiation therapy is not typically used as a primary treatment for cystosarcoma phylloides, although it may be used in conjunction with surgery for malignant tumors.
* **Option C:** Chemotherapy is not the standard treatment for cystosarcoma phylloides, as these tumors are often resistant to systemic therapy.

**Clinical Pearl / High-Yield Fact**
Phyllodes tumors can be challenging to diagnose, and a definitive diagnosis requires histopathological examination of the tumor. A key distinguishing feature of phyllodes tumors is their lack of ductal differentiation, which is in contrast to fibroadenomas.

**Correct Answer: C. Wide local excision with clear margins.**