**Core Concept**
Extramammary Paget's disease (EMPD) is a rare skin malignancy that affects the apocrine gland-bearing areas of the body, such as the vulva, penis, and perianal region. It is characterized by the presence of Paget cells, which are large, abnormal cells that produce mucin.

**Why the Correct Answer is Right**
EMPD is thought to arise from the intraepithelial involvement of Paget cells, which are believed to originate from the apocrine gland epithelium. The exact pathogenesis is unclear, but it is thought to be related to the malignant transformation of apocrine gland cells. The disease often presents with a slow-growing, eczematous, or ulcerative lesion in the affected area.

**Why Each Wrong Option is Incorrect**
**Option A:** EMPD is not typically associated with the axilla, which is more commonly affected by other conditions such as axillary hyperhidrosis or hidradenitis suppurativa.
**Option B:** While EMPD can involve the vulva, it is not the most common site of involvement; the vulva represents about 50-60% of cases.
**Option C:** EMPD is not typically associated with the axillary tail of the breast, which is more commonly affected by conditions such as breast cancer or gynecomastia.

**Clinical Pearl / High-Yield Fact**
EMPD is often mistaken for other benign or malignant conditions, such as eczema, psoriasis, or invasive carcinoma. A high index of suspicion and prompt biopsy are essential for accurate diagnosis.

**Correct Answer: D. Perianal region.**