**Core Concept**
Her-2/Neu immunohistochemistry is a diagnostic tool used to detect the overexpression of the human epidermal growth factor receptor 2 (Her-2/Neu) protein in breast cancer cells. This protein plays a crucial role in the development and progression of certain types of breast cancer.

**Why the Correct Answer is Right**
The Her-2/Neu protein is a tyrosine kinase receptor that, when overexpressed, can lead to increased cell proliferation and tumor growth. Immunohistochemical staining for Her-2/Neu is used to identify patients who may benefit from targeted therapies, such as trastuzumab, which can help to slow or stop the growth of the tumor. The staining is typically performed on a tissue sample obtained through a biopsy or surgical specimen.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Her-2/Neu immunohistochemistry is not used to diagnose breast cancer in general, but rather to identify a specific subgroup of patients who may benefit from targeted therapy.

**Option B:** This option is incorrect because Her-2/Neu immunohistochemistry is not used to detect the presence of estrogen receptors in breast cancer cells.

**Option C:** This option is incorrect because Her-2/Neu immunohistochemistry is not used to detect the presence of progesterone receptors in breast cancer cells.

**Clinical Pearl / High-Yield Fact**
Her-2/Neu overexpression is more common in invasive ductal carcinomas and is associated with a worse prognosis. However, targeted therapies such as trastuzumab have improved outcomes for patients with Her-2/Neu-positive breast cancer.

**Correct Answer:** D.