This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 1, Subject name: Physiology
Which of the following factors increases the resistance of the airways? 
Select and explain. 
A. Increasing lung volume above FRC, B. Increased sympathetic stimulation of airway smooth muscle, C. Going to high altitude, D. Inhaling cigarette smoke
D: Inhaling cigarette smoke
Answer: D: Inhaling cigarette smoke 
Explanation: Inhalation of cigarette smoke causes reflex constriction of airway smooth muscle as a result of stimulation of irritant receptors in the airway wall. The other choices are incorrect: Both increasing lung volume above FRC and sympathetic stimulation of airway smooth muscle reduce airway resistance. Going to high altitude does the same because the density of the air is reduced. The density is also decreased when nitrogen is replaced by helium in the inspired gas.