**Core Concept**
Nicotine withdrawal refers to the physical and psychological symptoms that occur when an individual abruptly stops or significantly reduces their nicotine intake after prolonged use. It is characterized by a complex interplay of neurotransmitter imbalances, particularly involving dopamine, serotonin, and acetylcholine.

**Why the Correct Answer is Right**
Nicotine withdrawal symptoms typically include irritability, anxiety, insomnia, increased appetite, and restlessness. These symptoms occur due to the sudden decrease in dopamine and acetylcholine levels in the brain, which are associated with the activation of nicotinic receptors. The withdrawal process also involves the upregulation of nicotinic receptors, making the individual more sensitive to nicotine.

**Why Each Wrong Option is Incorrect**
**Option A:** Insomnia is a feature of nicotine withdrawal. The correct answer is not insomnia.
**Option B:** Irritability is a feature of nicotine withdrawal. The correct answer is not irritability.
**Option C:** Weight gain is a feature of nicotine withdrawal due to increased appetite. The correct answer is not weight gain.

**Clinical Pearl / High-Yield Fact**
Nicotine withdrawal symptoms can be managed with non-nicotine medications such as bupropion or varenicline, which work by modulating nicotinic receptors and reducing cravings.

**Correct Answer: D. Weight gain is not typically a characteristic of nicotine withdrawal.**