**Core Concept**
Piloerection, also known as horripilation, is a physiological response characterized by the standing of hairs on end, often associated with fear, stress, or withdrawal from certain substances. This phenomenon is primarily mediated by the sympathetic nervous system, particularly through the release of certain neurotransmitters.

**Why the Correct Answer is Right**
Morphine is an opioid, and withdrawal from opioids can lead to a variety of symptoms, including piloerection. This occurs due to the sudden decrease in endogenous opioid levels and the subsequent activation of the sympathetic nervous system. The release of neurotransmitters such as noradrenaline and adrenaline from the hypothalamus and adrenal glands stimulates the arrector pili muscles, causing the hair follicles to contract and stand upright. This response is a classic sign of opioid withdrawal.

**Why Each Wrong Option is Incorrect**
**Option B:** Cannabis withdrawal does not typically cause piloerection, as it primarily affects the central nervous system's cannabinoid receptors rather than directly stimulating the sympathetic nervous system.

**Option C:** Smoking withdrawal does not lead to piloerection, as nicotine's effects are primarily mediated through nicotinic acetylcholine receptors in the central and peripheral nervous system, rather than directly causing a sympathetic response.

**Option D:** Alcohol withdrawal can cause a range of symptoms, including tremors, seizures, and anxiety, but piloerection is not a characteristic feature of alcohol withdrawal.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between opioid and non-opioid withdrawal symptoms, as the treatment approaches and management strategies differ significantly. Opioid withdrawal often requires more aggressive management, including the use of opioid replacement therapy and symptom-specific medications.

**✓ Correct Answer: A. Morphine. Morphine withdrawal causes piloerection due to the activation of the sympathetic nervous system.**