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NBME 21 Answers

nbme21/Block 2/Question#26 (44.1 difficulty score)
A healthy 32-year-old woman at 35 weeks' ...
Birth canal reflexπŸ”

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submitted by hayayah(1057),
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tctehrS or ailtonid fo the rcxevi dna ganiva rae rstogn lmiusti rfo oxnytioc nstre,cioe iameedtd yb rnulea spahtwya llcdea hte snoFurge lreexf.

ilA:tecr eoec/.c-wlriew:isrcisxrucceunt/.tcnefepso/woied/gepmefsohnntcrt/s

readit  This is also why ob-gyn's will massage the uterus (which is part of the birth canal) after delivery. It's to get the body to release oxytocin and cause the uterus to contract (to prevent postpartum hemorrhage) +5  
jennybones  Please why is estrogen not the answer, I thought estrogen would upregulate oxytocin receptors and increase oxytocin secretion? +1  
drzed  During pregnancy, oxytocin RECEPTORS are upregulated (by estrogen) as parturition approaches, but the ferugson reflex creates a positive feedback loop where the dilation of the cervix further releases more oxytocin. The inciting event that starts this feedback loop is the dilation of the cervix leading to a direct release of oxytocin, not the presence of more oxytocin receptors. +3  
bbr  To add, the positive feedback loop here is pretty elegant. Basically, it seems like the cervix doesnt want anything in it. When its stretched --> oxytocin --> contract uterus --> push baby --> more stretch at cervix---> more oxytocin (and forward). The positive feedback loop aspect is that oxytocin-based contractions cause more dilation of the cervix (as the baby moves into it!). +1  

submitted by medschoollovin(14),
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"Uopn iioappltcan fo sseurepr ot het rnieatnl end fo eht rvcx,ie otinyocx si eedalrse r(eferheto ransciee ni cnttlaiorec iset)ronp, wchhi leistastum renutei sotatroinnc,c hihcw ni rnut encesrias urespsre no eht ecrvxi be(rehty aginrescni cytooxin s,leeaer e.,ct) itnlu het byab si iddevreel.

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seagull a counter argument for PGE if you chose that answer. However, the author believes oxytocin is superior. +  
usmleuser007  1) PGE rises initially that causes the uterine contractions= this would be equivalent to when someone say #of contractions per time period. 2) Oxytocin is increased when the cervix is manipulated (ie. the birth canal reflex). +6  
jennybones  Please why is estrogen not the answer, I thought estrogen would upregulate oxytocin receptors and increase oxytocin secretion? +