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nbme24/Block 2/Question#12

An 80-year-old man is admitted to the hospital ...


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They’re giving a lot of confusing extra information here, maybe to trip us up. They just want volume of distribution, simple as that.

Vd = [drug administered] ÷ [plasma drug concentration]

First convert it all to g/L because this is how the answer will be:

administered: 80 mg = 0.08 g plasma concentration: 4 ug/ml = 0.004 g/L


Vd = 0.08 grams ÷ 0.004 g/L = 20 L

Clearance of drug is not a huge factor because the half life is so long that the drug is distributing before significant clearance occurs.

gonyyong  I think the distribution half-life and elimination half-life was saying that by the time you checked, it had fully distributed (10 half-lifes) and had not been cleared yet (super long half-life) +7  
soph  1000ug= 1mg and 1g=1000000ug so then 4ug/ml * 1g/ 1000000ug= 0.000004 g/ml 0.000004g/ml * 1000ml/L= 0.004 g/L 80mg*1g/1000mg= 0.08 g vd= 0.08g/ 0.04g/l =20L +3  
tiredofstudying  Or, like a normal human, convert 4ug/mL into mg/L ... which is 4 mg/L. 80mg/4mg/L is 20L. +1  

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submitted by fahmed14(18),

Vd = amount of drug in body / plasma drug concentration.

Amount of drug in body = 80mg

Plasma drug concentration = 4 micrograms/mL = 4mg/L

Vd = 80mg/(4mg/L) = 20L

Distribution half-life is 3 minutes so after 30 min we can assume the drug is maximally distributed. Elimination half-life is 24 hours so we can assume a neglible amount of drug lost

 +1  upvote downvote
submitted by lm4(6),

you could also have used the loading dose equation (because they gave you loading dose and told you it was IV) LD= (Vd x Cp)/ F if you freak out at converting units like I do