welcome redditors!to snoo-finity ... and beyond!

NBME 22 Answers

nbme22/Block 2/Question#45

An 86-year-old man who lives in a skilled nursing ...

Dextromethorphan

Login to comment/vote.

 +4  upvote downvote
submitted by rockediny(4),

Dextro is the correct answer here. From the choices given, dextro is the least likely to cause constipation since its main mechanism of action is NMDA antagonism w/some opioid activity -- it can cause constipation but the other choices are MUCH MORE likely to. As for diphenhydramine = it is not appropriate for elderly patients and it isn’t an antitussive.

forerofore  diphenhydramine and other Gen 1 antihistamines are good choices for chronic cough in allergy and patients with posterior draining and post viral chronic cough (careful with the elderly). That being said, apparently it also causes constipation, so dextro is still the correct answer. +  

 +3  upvote downvote
submitted by noselex(4),

Dextro vs Codeine: Both are antitussives, but dextro is more of NMDA agonist that also has opioid agonist activity. Dextro is often abused to get a similar out of body feeling due to its NMDA agonist effect. Codeine on the other other is a full-on opioid agonist. It’s also used as anti-diarrheal, so constipation is very common adverse effect.

Tiotropium is wrong because it’s not an antitussive. Also, it’s an anticholinergic which is (1) contraindicated in elderly unless really indicated, (2) a well-documented anticholinergic effect is constipation.

pparalpha  According to Sketchy and Amboss: dextromethorphan is a weak opioid receptor AGONIST and NMDA receptor ANTAGONIST (it's not an agonist). +  

 +1  upvote downvote
submitted by calcium196(6),

How is a synthetic opioid (dextromethorphan) that you can find with a 2 second google search as causing constipation the correct answer? Is it just because tiotropium wouldn’t treat the cough?

moneysacs  Tiotropium would cause constipation bc it’s an anticholinergic. Don’t have any insight into why dextromethorphan is the right answer though, other than process of elimination and diphenhydramine wouldn’t treat his symptoms. +  
upstairs_bumblebee  I thought it was dextromethorphan because it would bind NMDA receptors and have weaker opioid effect = less constipation? Idk though :/ +  
dr_salface  I agree. I mainly went by way of elimination but even DXM made me think of opiod-induced constipation. Though, all the other answer choices had obvious anticholinergic effects. At least, that was my reasoning for picking dxm. +  

 +0  upvote downvote
submitted by calcium196(6),

NBME 20 has a question with a guy taking over the counter cough medication and now he has constipation. Want to guess the answer? It was dextro! -> https://nbmeanswers.com/exam/nbme20/458

So I’m pretty sure the NBME 22 question is just straight up wrong.

rockediny  No, not so. Dextro *is* the correct answer here. From the choices given, dextro is the least likely to cause constipation since its main mechanism of action is NMDA antagonism w/ *some* opioid activity -- it can cause constipation but the other choices are MUCH MORE likely to. As for diphenhydramine = it is not appropriate for elderly patients and it isn’t an antitussive. +  
surely_not_a_robot_  Agreed with @rockediny. Dextro would be the best to prescribe because it has the least amount of constipation out of the drugs that you could prescribe + Anti-cholinergics in the elderly have much more morbidity and risk of mortality. +  
keycompany  The only way to wrap your head around this is to conclude that Dextro is the "least wrong". I thought a lot about this, and I can't think of any drugs that can suppress cough without also causing constipation, so it makes sense that Dextro is the answer because it is the "least likely" to cause significant constipation. This is probably just a clinical correlate that will be learned during rotations/years in practice. +  

 +0  upvote downvote
submitted by surely_not_a_robot_(0),

Per first aid. Dextromethorphan = “Antitussive (antagonizes NMDA glutamate receptors). Synthetic codeine analog. Has mild opioid effect when used in excess.”

I guess the key is that the opioid effect is mild. Seems like they’re asking which would treat the symptoms and reduce the side effects, not necessarily get rid of the side effects completely. Agree that it is a challenging one for Step 1.