What is the physical condition characterized by "hair standing up"?

The correct answer is piloerection, which refers to the condition where hairs on the body stand up, often triggered by various stimuli such as cold temperatures or strong emotions, like fear or excitement. This reflex action involves the contraction of tiny muscles at the base of hair follicles, making the hair stick upright. This reaction is a remnant of our evolutionary history, where raised hair would make animals appear larger and help with insulation in cold conditions.

Dermatitis refers to inflammation of the skin, which does not involve hair standing up. Hyperhidrosis is the condition of excessive sweating, unrelated to hair movement. Alopecia is a condition characterized by hair loss, so it also does not pertain to the phenomenon of hair standing up. Understanding these distinctions highlights the unique nature of piloerection as the correct choice in the context of physical responses.