**Core Concept**
Nevus achromicus and Nevus anemicus are two distinct types of congenital melanocytic nevi that present with different clinical manifestations. Nevus achromicus is characterized by a lack of melanocytes, resulting in a white or hypopigmented patch, whereas Nevus anemicus is caused by an abnormal vascular response, leading to a localized vasoconstriction and resulting in a pale or anemic-appearing patch.

**Why the Correct Answer is Right**
The key to differentiating between these two conditions lies in their response to cold stress or emotional stimuli. Nevus achromicus remains unchanged, whereas Nevus anemicus exhibits a characteristic blanching or darkening in response to these stimuli due to the vasoconstriction. This is because Nevus anemicus is a result of an abnormal adrenergic receptor-mediated vasoconstriction, typically involving alpha-2 adrenergic receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe a characteristic that differentiates Nevus achromicus from Nevus anemicus.
**Option B:** This option is incorrect because it is not a reliable method for distinguishing between these two conditions.
**Option C:** This option is incorrect because it does not accurately describe a key feature that differentiates Nevus achromicus from Nevus anemicus.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Nevus anemicus often presents with a characteristic "blanching or darkening" response to cold stress or emotional stimuli, which is not seen in Nevus achromicus.

**Correct Answer: C.**