This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Medicine
Varenicline is used in 
Select and explain. 
A. Pulmonary hemosiderosis, B. Sleep apnea, C. Anti-trypsin deficiency, D. Nicotine dependency
D: Nicotine dependency
Answer: D: Nicotine dependency 
Explanation: Varenicline is a prescription medication used to treat smoking addiction. This medication is the first approved nicotinic receptor paial agonist. Specifically, varenicline is a paial agonist of the alpha4/beta2 subtype of the nicotinic acetylcholine receptor Ref Davidson 23rd edition pg 578