**Core Concept**
Nevus anemicus is a rare congenital vascular anomaly characterized by a localized area of pale skin due to vasoconstriction. This condition is caused by an abnormality in the vascular regulatory mechanisms that control blood flow to the skin.

**Why the Correct Answer is Right**
The underlying pathology of nevus anemicus involves the abnormal function of the sympathetic nervous system, which normally regulates blood flow to the skin. In this condition, the sympathetic nerves release excessive amounts of vasoconstrictors, such as norepinephrine, leading to sustained vasoconstriction and decreased blood flow to the affected area. This results in the characteristic pale appearance of the skin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because nevus anemicus is not caused by a lack of blood vessels or hypoplasia of the skin.
**Option B:** This option is incorrect because nevus anemicus is a congenital condition, not an acquired one caused by inflammation or trauma.
**Option C:** This option is incorrect because nevus anemicus is not caused by abnormal melanin production or a defect in melanocyte function.

**Clinical Pearl / High-Yield Fact**
Nevus anemicus is often misdiagnosed as a port-wine stain or a vitiligo-like condition, but it can be distinguished by its characteristic vasoconstriction response to cold temperatures or emotional stimuli.

**Correct Answer: C. Abnormal sympathetic vasoconstriction**