**Core Concept**
Mongolian spots are benign flat congenital melanocytic lesions that typically appear at birth or within the first few weeks of life, most commonly on the lower back, buttocks, or sacrum. They are caused by an accumulation of melanin in the skin due to increased melanocytes in the dermal-epidermal junction.

**Why the Correct Answer is Right**
Mongolian spots are characterized by their presence in individuals of African, Asian, or Native American descent, although they can occur in individuals of any ethnic background. The lesions are usually oval or circular in shape and range in color from blue-gray to dark brown. They are typically flat and may fade or disappear by the age of 5-6 years. The exact pathophysiology is not fully understood, but it is thought to be related to the migration of melanocytes during fetal development.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Mongolian spots are not typically associated with an increase in the number of melanocytes in the epidermis, but rather in the dermal-epidermal junction.
**Option B:** This option is incorrect because Mongolian spots are not a form of melanoma or other malignant skin cancer.
**Option C:** This option is incorrect because Mongolian spots are not usually painful or tender to the touch, and are typically not associated with any underlying systemic disease.

**Clinical Pearl / High-Yield Fact**
Mongolian spots are a common benign finding in infants and children, and are often seen in conjunction with other congenital skin lesions such as café-au-lait spots or nevus flammeus.

**Correct Answer: D.**