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NBME 20 Answers

nbme20/Block 3/Question#23 (76.7 difficulty score)
A 50-year-old woman is brought to the ...
pKa: 6.0; Protein binding in plasma: 20%; Volume of distribution at steady state: 1 L/kg🔍,📺

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submitted by hayayah(1100),
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nI deorr rfo a urdg to be ceaelrd yb het dykein, it mtsu isftr be frldteie in the rui.ollegm sDugr thiw a gihh DV ahve orme fo teh gudr in teh eustsi that aer ton vallbaiae to rfteledi by hte ed.yikn rgDus ihtw high trniope dngibin wno't eb lritdefe ie.etrh oS oyu want a udgr wthi owl Vd nda lwo ninidbg if uyo wnat ti cdeearl via teh kdnsyie and euirn.

zevvyt  But a low VD corresponds to high Plasma Binding Concentration(FA 233, 2019). That's my main confusion with this question. +2  
kevin  If it's high plasma binding, then it's low Vd. But, low VD doesn't necessarily require high plasma binding. Low Vd can simply be due to it being a large polar molecule +1  
topgunber  So you the confusion is that VD of 1 means it stays in the circulation/ intravascular compartment. This would make you think that it would bind highly to protein in because of the low VD. But im assuming if it bound proteins it would not be cleared by the kidneys, so if this had a low VD (can be cleared at glomerulus) and if it bound plasma proteins weakly (allows the kidney to filter the substance) then it can be filtered into the tubular lumen. Now the pka. Bases when protonated (ph under their pka) become charged, while acids at ph under their pka get protonated are UNCHARGED, meaning in acidic urine acids become more lipophillic and can diffuse out of the tubular lumen. If we made the urine more basic, then we want to deprotonate the acid (ph higher than pka) so it will remain stuck in the tubular lumen as an ion. +  

submitted by charcot_bouchard(477),
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aKp si pH ta hicwh nay ugdr si ta its 50% idneioz

woN we aer alk rnuie e.i cni p.H ehwn p;aH&gptK ti illw ehva owt idff apht ofr dcciai grud ;& sicab udg.r

icdiAc drug will nci tis mitnainiloe ni(c dioinze ,m)orf ascbi ugrd will eb orem ssba.robo ed ew eden to wokn eth rgud is bcasi iic/dca.

woN fi u lka erniu sti iioamelinnt ci.n so ti hvea to eb icaic.d ro u can nokw ist a osid tsla of grud wiht CNS reypotrp ie. sotm lkie lPaioerbhanbt (Wkea )cdai

os fi Kap of gdur si -ta6-- pH 7 ew lliw attrs naeilntgiim

utb fi Kap is 0 we need to irsae pH fo uiren at 11 ot ttsar inetlgntai.ami ahtt ptnio vper gdru aK6=p() dwolu be tayotll tuo of my.etss

httsa why A si teh itrgh nas aKp( = 6)

charcot_bouchard  Correction : Not 0. i means if pKa is 10 +2  
charcot_bouchard  2nd update : cont to learn school grade chem. if pKa > 7 it is base. and if pKa is < 7 it is acid. Since we established the drug have to be a weak acid pKa cant be more than 7. +  
dbg  thanks, but Pka and PH are not at all the same thing +2  

submitted by monoloco(141),
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fI uoy ntwa to elarc a rugd, it is yalbpobr etsb taht it tno be dubon ot esnitopr os( ahtt ti tesg tdreef)li nda ti hsa a wol uevmlo of ibsiuntotird so( it tni's ni teh p,ede dhra ot erhac iet.uss)s

kingtime9119  But that doesn't make sense. Page 233 of First Aid 2019 edition clearly states that being plasma protein bound creates the lowest volume of distribution, because not being bound to proteins increases the chance it will reach deep into the tissues before it reaches the kidneys. Discrepancy with First Aid? +  
haliburton  my reasoning was comparing two drugs, both with Vd of 1, the drug with the lower albumin binding would be cleared faster @kingtime. I don't think you're considering that A and B have equal Vd. +8  

submitted by lispectedwumbologist(109),
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anC someyodb aeslpe nxeialp hyw het Kpa hsa to eb 6 enaitds of ?01

masonkingcobra  Since an ionized form is charged (by definition), it will not easily cross a nonpolar lipid membrane. Thus, it is important to recognize the potential of the drug to ionize in order to predict its solubility and the degree to which it can be reabsorbed. The degree of ionization is determined by the drug’s pKa and the pH of its environment. Weak acids and bases are 50% ionized and 50% unionized when the surrounding pH equals the drug’s pKa. At 2 pH units above or below the pKa of the drug, nearly 100% of the drug is ionized or unionized. +3  
masonkingcobra  Basically weak acids are best excreted in alkaline urine, but weak bases are excreted more readily in acid urine. +  
masonkingcobra  In summary, because this is a weak acid at pKa 6, making the urine alkaline will result it its ionization and excretion. Ionized cant move through lipid membranes so can't get reabsorbed and is pissed out. +17  
yex  Following on masonkingcobra explanation: A pKa 4-9 can be either weak acid or base. Weak acid pKa 4-7; strong acid pKa 1-3 Weak base pKa 7-9; strong base pKa above 9 Differents pHs: stomach: 1-2 duodenum: 3-5 early jejunum: 5-7 late jejunum: 7-9 ileum: >9 urine: 4.5-8 Weak acids (pKa) gets absorbed in acidic (pH) environments and cleared in basic. Weak bases gets absorbed in basic environments and cleared in acidic. THIS DOES NOT APPLY TO STRONG BASES OR ACIDS!!!! The best explanation for this is a Biochem lecture from Pass Program and it is available on YouTube, its long but it is for sure worth it!! Look for 19 Biochemistry 1 from Pass Program on YouTube. +2  

submitted by chandlerbas(106),
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rlhatig etls od shi.t osem of hte cetmsnmo ewre lyrela lelw ,lenxdipea owrehve im oging ot ryt aorenth r.otue ltse ldae thwi Kap nsiec eht hetro ecnmomst nxepaedila dV wlel gnouh.e kodus l./fsesflleola

itsfr tnigh is yuo heav to nmedertie fi sit na caid or .beas %90 fo the miet tsi noigg ot eb a waek di,ca ew lipmse dont fmoleautr uchm acmstoednii sa aebss in er.gelan lsoa ist sdimou atsl dna CNS os you wokn ti sha to be opebrhait.blan teps 1() wol paK easmn adic -47.)( oyu ustm konw hst.i ()2 uyo sumt wonk atth all adsci sratt off anrutle adn hetn oles an oin &t-;g- icgembno gda.rche dna lal asesb trtas ffo raegdch nad nteh oesl an oin to cbeeom eltraun (hits si hwy ew ntdo ues asbci urgds cuesbea isht unrtlae rhcage sha a hgih vd nad spotdise ni sessitu = bad in leldyr).e uyo nawt ceagdrh cb ahstt htwa si dhoihcriply aka epe it out. (3) enxt esu wtah i jjstu adsi dan yaplp ti ot tsih ratch:

2+ = %99

1+ = 09%

1- = 1%0

-2 = 1%

het fsrit rmeunb eetrnrepss eht iefcrnfede ewenbet Hp adn pKa (aKp si ivgen by the isentquo ,ylwasa) pH is wtah you hndaecg whne you edlzlaniak teh urnie. os for hist inotesuq ti lduwo be: paK-pH = hhig sabic lauve )7t&(;g insmu kpa ( eakw cida .7-)4

onw teh sconde rtceneapge tnrpsseeer teh eenptgcare of dtsiacoedsi toonpr. os rfo na idca, eyth lwil cmeobe hegcdra, tish esamn hatt if hte refeicfend nteeewb hte ph nda apK is orev 1+ ethn 0%9 fo hte acdi will go rfmo trnaule ot eagrdh.c fi the hp si 2+ rgeeatr hatn pAK enth 99% of eth cdai wlli veha tlos ist otpron nda eeobmc gdhcare

tles zamrsmiue tih:s ewre dtol tis a kewa daic = aka Kpa etwnebe 7.4- we atnw ot etg to atek eht akew aidc fmor sti nltuera ofrm into a ardegch ofmr ot gte rdi of .ti os ew niklazleaa hte ,unier ceabeus abses aer erhigh pH uoy will get a omre + benurm hstu hte kwea aidc llwi go mfro utnarel ot rdeagch nda be e.rdetxec

submitted by strugglebus(172),
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I swa wnidrngeo if it swa to lowal ofr tlaapir SNC toeenrptnia iecsn teh urdg is dlsoeppyus pdeussop to vhea SCN etcffe

submitted by sha2507(0),
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rfo ekwa ksipHca=dp a + log -/()A)(HA kaa nehsdreon hcsshaeabl fni tieaouq oyu elsvo ist,h 'ylluo gte atth pH - kap = gd(Ao H(anA-))l/ htne iaagn yuo llwi teg 10 ot weorp ph - kpa = -/A.AH Rfee[r ot inkl ni mmnwe]ston co ndneigcoris ttha eth ph fo ruo odby si 37. e ein,twmogsh acn vlsoe ofr akp 10 adn kap 6 sguni hist Tamo tlar.uhf si for apk 6 nad bdyo ph 3.,7 it lwli eb 01 to the oerpw of ,3.1 owldu mnea thta AH/A roati si a teovpisi urn,ebm piyilmgn htta A- &g;t A.H hesrewa ofr nwhe pak si ,01 teh /A-HA onrtai si 01 to prweo of .-27, neamgin tath -Hg.tA;A&

dAn as earlday tondeemin ni hte avboe mom,tnces ew dnee na icicno ,mrfo ot altienime eht udgr, it oluwd eb pdreeferr fi teh pka si 6 as ni hatt a,sec )(icA-oin lowud be in reom rannonc.coeitt

sha2507 refer to this video, to understand it a little better, and for the formula with the table, skip to 7:26 +  

submitted by sway(0),
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os thwa fi ihst aws senidevr rhewe the grdu ni teosnuqi wsa a sbea dna hatt niure saw f.adeidicii wkon teh apK uowdl dnee to be &7;tg btu htwa atuob the pH

submitted by kingtime9119(0),
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oem[tmnc mdevo to mubntos]emc

submitted by haliburton(217),
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cmone[mt vmdoe ot steom]nmcbu