**Core Concept:** Varenicline, also known as Champix, is a medication used for the treatment of nicotine addiction. It is a partial agonist at the α4β2 nicotinic acetylcholine receptors and a competitive antagonist at the α4β2 receptors.

**Why the Correct Answer is Right:** Varenicline acts by selectively binding to α4β2 nicotinic acetylcholine receptors, which are found on the surface of neurons in the central nervous system. When varenicline binds to these receptors, it mimics the effects of nicotine but with a lower potency, leading to reduced craving and withdrawal symptoms associated with nicotine withdrawal. Additionally, varenicline blocks the activation of α4β2 receptors by nicotine, reducing the reinforcing effects of smoking and increasing the aversiveness of smoking.

**Why Each Wrong Option is Incorrect:**

A. **Nicotine receptor antagonist:** While varenicline does have nicotine receptor antagonist properties, the correct answer is not this option. Varenicline's primary mode of action is through partial agonism at α4β2 receptors, not simple antagonism.

B. **α4β2 receptor partial agonist:** While varenicline does act as a partial agonist, the correct answer is not this option. Varenicline's primary mode of action is through blocking nicotine's action at α4β2 receptors, not acting as a partial agonist itself.

C. **Nicotine receptor antagonist:** As mentioned earlier, varenicline's primary mode of action is through partial agonism at α4β2 receptors, not simple antagonism.

D. **α4β2 receptor partial agonist:** As explained earlier, varenicline's primary mode of action is through blocking nicotine's action at α4β2 receptors, not acting as a partial agonist itself.

**Clinical Pearl:** Varenicline's dual action at α4β2 receptors helps in reducing smoking cessation relapse rates, as it targets nicotine addiction at both the neuronal and behavioral levels.

**Correct Answer:** C. α4β2 receptor partial agonist

Varenicline acts as a partial agonist at α4β2 nicotinic acetylcholine receptors, which are mainly present in the central nervous system. This action reduces the reinforcing effects of nicotine, leading to decreased cravings and withdrawal symptoms in smokers attempting to quit. By activating α4β2 receptors, varenicline mimics the effects of nicotine, although with lower potency. This makes smoking less rewarding and less reinforcing, which helps in smoking cessation.