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NBME 22 Answers

nbme22/Block 1/Question#41

A 21-year-old man is brought to the emergency ...

Gender and body habitus

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 +4  upvote downvote
submitted by drdoom(162),

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 that person?” Or you can phrase it these ways:

  • Given a CONDITION (spontaneous pneumo), what other finding is most likely to be the case?
  • 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? It’s the latter.

someduck3  Is this the best approach to all of the "strongest predisposing risk factor" type questions? +  
drdoom  There is a town of 1,000 men. Nine hundred of them work as lawyers. The other 100 are engineers. Tom is from this town. He rides his bike to work. In his free time, he likes solving math puzzles. He built his own computer. What is Tom's occupation most likely to be? Answer: Tom is most likely to be a lawyer! Don't let assumptions distract you from the overwhelming force of sheer probability! "Given that Tom is from this town, his most likely occupation (from the available data) = lawyer." +2  
drdoom  There is a town of 1,000 spontaneous pneumo patients. Six hundred are tall, thin and male. The other 400 are something else. Two hundred of the 1,000 smoke cigarettes. The other 800 do not. What risk factor is most strongly associated with spontaneous pneumo? (Answer: Not being a smoker! ... because out of 1,000 people, the most common trait is NOT smoking [800 members].) +3  
impostersyndromel1000  this is WILD! thanks guy +3  
belleng  beautiful! also, i think about odds ratio vs. relative risk...odds ratio is retrospective of case-control studies to find risk factor or exposure that correlates with grater ratio of disease. relative risk is an estimation of incidence in the future when looking at different cohort studies. +  
drdoom  @impostersyndrome I love me some probability and statistics. Glad my rant was useful :P +  
hyperfukus  @drdoom i hate it which is why your rant was extremely useful lol i learned a ton thanks dr.doom! +1  

 +2  upvote downvote
submitted by drdoom(162),

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? It’s the latter.


 +2  upvote downvote
submitted by seagull(349),

Why spontaneous? He's engaging in an active sport with an increased risk of Traumatic injury. So we really just assume hes not injured because the stem doesnt directly say he's injured? These questions lead to too many assumptions. (in my opinion)

nc1992  Spontaneous pneumothorax, as a condition, is significantly more likely than a traumatic pneumothorax from just about anything but a car crash (ok maybe if he was in a fight). The car crash or a stabbing is also more probable overall but there's no point in inferring something that isn't provided +  
nwinkelmann  I picked the traumatic injury also. After reading these comments I looked into it further. Traumatic pneumos occur because of blunt or penetrating chest trauma, and I found that the MCC form of blunt trauma (>70%) is motor vehicle acidents that cause significant trauma (i.e. rib fractures) or even blast trauma. Although it didn't say there were no chest wall fractures, at the same time it didn't indicate any rib fractures, which would be most like to cause the traumatic injury pneumo in the patient's case. +1  
drdoom  The stem makes no mention of trauma. +  
hyperfukus  i guess the issue is that you have to assume what they mean by "strongest predisposing risk factor for this patient's condition" I think this is dumb bc the answer is completely different based on what you consider this patient's "CONDITION" to be? either way he has a pneumothorax so if you wan to know what caused that its prob him being active or bumping into someone but if you consider the etiology of the pneumothorax then its the bleb and that is from him being a skinny dude/smoker i went to this b/c he's also only 5/10 that's not tall in my head they could have been nicer and made him 6'1 at least...also i feel like i saw a lot of q's back in the day when i first learned this with a presentation of the person like tripping or something dumb but they already had the bleb and then got the pneumothorax +  

 +1  upvote downvote
submitted by cocoxaurus(22),

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.


 +1  upvote downvote
submitted by nwinkelmann(54),

Here's some epidemiology information about spontaneous pneumos: There are two kinds, primary spontaneous pneumos (PSP) and secondary (SSP). PSPs occur in people aged 20-30 years, with a peak incidence is in the early 20s, is rarely observed in people older than 40 years, with a male to female ratio of 6.2:1, and most frequently occurs in tall, thin men, though smoking increases risk even further. SSPs occur in patients with underlying lung conditions, such as COPD/older patients. If the patient's demographic was older and had history of lung disease, but that wasn't an option, then I think smoking would be the appropriate choice because smoking is the biggest risk factor for underlying lung conditions.

Regarding the trauma, traumatic injury pneumos most commonly occur due to motor vehicle acidents where there is injury (i.e. fracture) to chest wall, and with blast injuries. Although the question didn't specify the chest wall was void of fractures, it also didn't say fractures were present, so I think it would be safe to assume that the collision injury didn't have enough force to cause the pneumo, especially with keeping in mind the epidemilogy of primary spontaneous pneumos.

At least, this is my thought process having researched it more, now.


 +0  upvote downvote
submitted by hello(49),

Patient has a hemithorax + CXR shows trachea deviates towards the hemithorax --> most likely diagnosis is spontaneous pneumothorax.

Epidemiologically, spontaneous pneumothorax is most associated with thin males

hello  Wait, maybe this is wrong logic? Please correct me regarding the exam findings if needed. +  
hello  Can confirm that this explanation is INCORRECT regarding trachea. Disregard this explanation. +  

 -1  upvote downvote
submitted by its_raining_jimbos(8),

Everywhere I found (UpToDate and several papers) said the smoking is the biggest risk factor for spontaneous pneumothorax, with body habitus and gender being a lesser risk. Am I just completely misunderstanding the question?

imresident2020  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. +  
drdoom  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. +  
cocoxaurus  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. +