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Yes smoking is a risk factor but not the best option among the choices given. Check FA, it says that it occurs more in tall thin young males. Smoking isn’t even mentioned. Tall & thin males are more at risk because they have more negative intrapleural pressure. Check Uworld for this.
You have to think about this using the concept of CONDITIONAL PROBABILITY. Another way to ask this type of question is like this: “I show you a patient with spontaneous pneumothorax. Which other thing is most likely to be true about this patient?” Said a different way: Given a CONDITION [spontaneous pneumo], what other finding is most likely to be the case? Still other words: Given a pool of people with spontaneous pneumothorax, what other thing is most likely to be true about them?
In other words, of all people who end up with spontaneous pneumo, the most common other thing about them is that they are MALE & THIN. If I gave you a bucket of spontaneous pneumo patients -- and you reached your hand in there and pulled one out -- what scenario would be more common: In your hand you have a smoker or in your hand you have a thin male? The latter.
Rupture of pulmonary blebs are a common cause of spontaneous pneumothorax in young adult males that are tall and thin. I know it's also associated with smoking, but gender and body habitus seemed like the more likely answer here since the patient is a young male.