**Core Concept:** Breast cancer is a type of cancer that starts in the cells of the breasts. Breast cancer can involve only one breast (unilateral) or both breasts (bilateral). Understanding the difference between unilateral and bilateral breast cancer is crucial in determining treatment options and prognosis.

**Why the Correct Answer is Right:** Bilateral breast cancer refers to cancer that affects both breasts simultaneously. In this case, the cancer has likely spread from the primary site, most commonly the mammary ducts or lobules, to both breasts. Treatment options for bilateral breast cancer may differ from those for unilateral breast cancer due to the presence of cancer in both breasts.

**Why Each Wrong Option is Incorrect:**

A. Unilateral breast cancer (also known as lobular carcinoma in situ or lobular carcinoma) occurs when cancer starts in the breast tissue but does not spread to other parts of the body. This type of cancer involves one breast only.
B. Metastatic breast cancer, also known as secondary cancer, refers to cancer that has spread from the primary site (usually the breast) to other parts of the body but not both breasts.
C. Triple-negative breast cancer (TNBC) is a type of breast cancer that does not express certain proteins and receptors, but it does not specifically refer to bilateral involvement.
D. HER2-positive breast cancer refers to cancer cells that have too many HER2 proteins, but it also does not specifically imply bilateral involvement.

**Clinical Pearl:** Understanding the differences between unilateral and bilateral breast cancer is critical for accurate diagnosis, treatment planning, and prognosis.

**Correct Answer:** B. Metastatic breast cancer refers to cancer that has spread from the primary site (usually the breast) to other parts of the body but not both breasts. This distinction is essential in determining treatment options and prognosis for patients with breast cancer.