**Core Concept**
The patient in question has undergone a mastectomy for stage IIB breast cancer, which indicates the presence of multicentric disease and axillary lymph node involvement. Distant metastasis is a potential complication in breast cancer patients, and understanding the most common sites for metastasis is crucial for clinical management.

**Why the Correct Answer is Right**
Breast cancer commonly metastasizes to bones, lungs, liver, and brain. However, the most common site for distant metastasis in breast cancer is the bone marrow, specifically the axial skeleton, including the vertebrae, ribs, and pelvis. This is due to the hematogenous spread of tumor cells, which are carried by the bloodstream to the bone marrow. The bone microenvironment provides a fertile ground for tumor cells to proliferate and establish metastatic lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** While the lungs can be a common site for metastasis in various cancers, they are not the most common site for distant metastasis in breast cancer.

**Option B:** The liver can be involved in breast cancer metastasis, but it is not the most common site.

**Option C:** The brain can be a site for metastasis in breast cancer, particularly in cases with HER2-positive or triple-negative breast cancer subtypes, but it is not the most common site.

**Clinical Pearl / High-Yield Fact**
The most common sites for distant metastasis in breast cancer are bone (70%), lung (20%), liver (5%), and brain (5%). This knowledge is essential for clinical management, as metastatic disease can significantly impact the prognosis and treatment approach.

**Correct Answer:** C.