**Core Concept**
Paget's disease of the nipple is a rare form of breast cancer characterized by the presence of malignant cells in the skin of the nipple and, sometimes, the areola. It is often associated with underlying ductal carcinoma in situ (DCIS) or invasive breast cancer.

**Why the Correct Answer is Right**
The malignant cells in Paget's disease of the nipple typically stain positive for cytokeratin 7 (CK7) and cytokeratin 19 (CK19), which are markers of epithelial origin. These cells are also often positive for estrogen receptor (ER) and progesterone receptor (PR), indicating a hormone-sensitive tumor. The presence of these markers helps in diagnosing Paget's disease and distinguishing it from other skin conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while cytokeratin 5/6 (CK5/6) is a marker of basal cells, it is not typically positive in Paget's disease of the nipple.
**Option B:** This option is incorrect because while epithelial membrane antigen (EMA) can be positive in Paget's disease, it is not as specific as CK7 and CK19.
**Option C:** This option is incorrect because while gross cystic disease fluid protein-15 (GCDFP-15) is a marker of apocrine cells, it is not typically positive in Paget's disease of the nipple.

**Clinical Pearl / High-Yield Fact**
Paget's disease of the nipple often presents with eczematous changes of the nipple and areola, which can make it challenging to diagnose. A high index of suspicion and prompt biopsy are essential for accurate diagnosis and treatment.

**Correct Answer: D. Cytokeratin 7 (CK7) and cytokeratin 19 (CK19).**