Step 1: Paget's disease outside the breast targets skin rich in apocrine glands, so the answer must be the genital site that is true skin rather than mucosa.
Step 2: The vulva is covered by keratinised, apocrine-bearing skin, which is exactly the soil for extramammary Paget's disease. It shows up as a persistent red, scaly, itchy plaque often mistaken for dermatitis.
Step 3: The vagina, cervix and uterus are mucosal surfaces lacking this apocrine skin, so they are not the recognised home of this lesion.
\[\boxed{\text{Vulva}}\]