**Core Concept**
Piloerection, also known as horripilation, is a physiological response characterized by the standing of hairs on end, often in response to cold temperatures, fear, or stress. This phenomenon is mediated by the sympathetic nervous system and involves the contraction of arrector pili muscles attached to hair follicles.

**Why the Correct Answer is Right**
The withdrawal of **noradrenaline (norepinephrine)** from the hair follicle leads to the relaxation of arrector pili muscles, causing the hair to stand upright. This is due to the inhibition of beta-2 adrenergic receptors, which are responsible for the contraction of these muscles. The reduction in noradrenaline levels results in decreased sympathetic tone, allowing the arrector pili muscles to relax, and the hair to return to its normal position.

**Why Each Wrong Option is Incorrect**
* **Option A:** Withdrawal of **acetylcholine** would not cause piloerection, as it is primarily involved in parasympathetic responses and muscle contraction, rather than hair follicle function.
* **Option B:** Withdrawal of **serotonin** is not directly related to piloerection, as serotonin is primarily involved in mood regulation, appetite, and sleep.
* **Option C:** Withdrawal of **dopamine** would not cause piloerection, as dopamine is primarily involved in reward, motivation, and motor control, rather than hair follicle function.

**Clinical Pearl / High-Yield Fact**
Piloerection is a key clinical sign of sympathetic nervous system activation, often seen in response to stress, fear, or cold temperatures. It is an important diagnostic clue in various medical conditions, including anxiety disorders and hypothermia.

**Correct Answer: C. Withdrawal of acetylcholine would not cause piloerection, as it is primarily involved in parasympathetic responses and muscle contraction, rather than hair follicle function.