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NBME 20 Answers

nbme20/Block 1/Question#46 (38.4 difficulty score)
A 55-year-old woman comes to the physician ...

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submitted by hayayah(1056),
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o,Ncite eht smte yass oeo"srpscrr ni hte ksin"

D3 ecricca)llfhooe(l mofr xeesourp fo sink a(tmtrus asl)bae to uns, iitgenosn of sih,f mikl, p.ltsan

D2 faceri)gcoerl(ol rmof intoinesg of al,ntps nu,fgi s.easyt

otBh rdteecvon to 2O5-H D3 teasog(r o)mrf in verli nad ot the vaiect rmof 51,2)H(2O- D3 tioa)(llcrci in .ikeydn

sympathetikey  C is the 3rd letter in the alphabet. Hence, D3 = Cholecalciferol +4  
karljeon  Thanks for the explanation. The question stem made it sound like "what future step will be decreased?" Actual question: "Decreased production of which... is most LIKELY TO OCCUR in this patient?" Maybe NBME needs a grammar Nazi working for them. +8  
bharatpillai  question says "decreased production of which of the following precursors in skin is most likely to occur in this patient? the answer has to be 7-dehydrocholecalciferol! +4  
bharatpillai  7 dehydrocholesterol +2  
brbwhat  Yeah i did the same, but then realised acc to uw flowchart 7dehydrochole.. is converted to cholecalciferol in presence of uv rays. So the decreased precursor would be cholecalciferol since we already have 7 dehydrocholecalciferol not being converted by uvrays Tho the uw chart sites both ergo and chole as dietary sources. +2  
drzed  Wouldn't 7-dehydrocholesterol build up in the skin? Since UV rays convert 7-dehydrocholesterol into cholecalciferol, if you are lacking the conversion, the reactant (7-dehydrocholesterol) should accumulate. +  
brbwhat  They’re asking decreased production of which of the following precursor would occur? 7 dehydrocholestrol builds up, but decreased production of cholecalciferol takes place, which is a precursor in the pathway for vitamin d formation +1  

submitted by baja_blast(91),

The key bit in this question that makes the answer Cholecalciferol rather than 7-dehydrocholesterol is the nugget that the patient "rigorously avoids exposure to the sun." Conversion of 7-dehydrocholesterol to Cholecalciferol can only occur when the skin is exposed to sunlight. Therefore, decreased production of Cholecalciferol (D3) is the better answer.

This picture always helps me remember Vitamin D metabolism.