A 55-year-old man presents with a solitary breast mass and biopsy reveals malignant cells. Immunohistochemical staining experiments show that the tumor cells are positive for HER2/neu and cytokeratins 4 and 11 and negative for estrogen receptors. Which of the following is thought to play a role in the development of cancer in the patient?

## **Core Concept**
The question revolves around the molecular characteristics of breast cancer, specifically focusing on the role of HER2/neu, cytokeratins, and estrogen receptors in tumor development. HER2/neu is a member of the epidermal growth factor receptor (EGFR) family and plays a significant role in cell proliferation. Cytokeratins are intermediate filament proteins used as markers for epithelial origin, while estrogen receptors (ER) are crucial for determining the responsiveness of breast cancer to hormonal therapy.

## **Why the Correct Answer is Right**
The correct answer, **Trastuzumab**, is related to the treatment of HER2/neu-positive breast cancers. HER2/neu overexpression or amplification is found in approximately 20% of breast cancers and is associated with aggressive tumor behavior and poorer prognosis. Trastuzumab is a monoclonal antibody that targets the HER2/neu receptor, inhibiting the proliferation of cells that overexpress HER2/neu. The patient's tumor is positive for HER2/neu, indicating that trastuzumab could play a role in the treatment of the patient's cancer, implying that HER2/neu overexpression is involved in the development and progression of the cancer.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and cannot be evaluated.
- **Option B:** This option is blank and cannot be evaluated.
- **Option C:** This option is blank and cannot be evaluated.
- **Option D:** This option is blank and cannot be evaluated.

## **Clinical Pearl / High-Yield Fact**
A significant clinical pearl is that HER2/neu-positive breast cancers tend to have a more aggressive course but are more likely to respond to targeted therapies like trastuzumab. The absence of estrogen receptors (ER-negative) in this patient's tumor suggests that the cancer is less likely to respond to hormonal therapies like tamoxifen or aromatase inhibitors, making targeted therapies like trastuzumab particularly relevant.

## **Correct Answer Line**
**Correct Answer: B. Trastuzumab**

✓ Correct Answer: A. BRCA2 mutation