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#1
03-29-2013
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Five Pharmacokinetics Questions!

1) 400 mg of a drug is administered to a patient and the drug is later measured in plasma to be 1 µg/ml. What is the apparent volume of distribution (Vd)?
a) 0.04 L
b) 0.4 L
c) 4 L
d) 40 L
e) 400 L

2) If a drug with a 2-hour half life is given with an initial dose of 8 mcg/ml, assuming first-order kinetics, how much drug will be left at 6 hours?
a) 8 mcg/ml
b) 4 mcg/ml
c) 2 mcg/ml
d) 1 mcg/ml
e) 0.5 mcg/ml

3) What percentage of the steady-state drug concentration is achieved at 3.3 * t(1/2)?
a) 10%
b) 25%
c) 50%
d) 75%
e) 90%

4) A target concentration of 7.5 mg/L of theophylline is required for a 60 kg patient. What is the loading dose, given the following: Vd = 0.5 L/kg, Cl = 0.04 L/kg/hr, t(1/2) = 9.3 hr?
a) 0.5 L/kg * 60 kg * 7.5 mg/L = 225 mg/h, infusion
b) 0.5 L/kg * 60 kg * 7.5 mg/L = 225 mg, bolus
c) 0.04 L/kg/hr * 60 kg * 7.5 mg/L = 18 mg/h, infusion
d) 0.04 L/kg/hr * 60 kg * 7.5 mg/L = 18 mg, bolus

5) A target concentration of 7.5 mg/L of theophylline is required for a 60 kg patient. What is the steady state maintenance dose, given the following: Vd = 0.5 L/kg, Cl = 0.04 L/kg/hr, t(1/2) = 9.3 hr?
a) 0.5 L/kg * 60 kg * 7.5 mg/L = 225 mg/h, infusion
b) 0.5 L/kg * 60 kg * 7.5 mg/L = 225 mg, bolus
c) 0.04 L/kg/hr * 60 kg * 7.5 mg/L = 18 mg/h, infus

#2
03-29-2013
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 161 Times in 85 Posts Reputation: 171

1) 1000 micro grams in 1 gram. i think that's the trick here. so convert everythiing to micrograms:

400,000/1,000 = 400,000 ml = 400L E

2) so after the first half life, we've got 4, after the second, we've got 2, and then after the third (which is 6 hours), we're at 1. D

3) I'm not sure if i understand the question. is it saying what percentage remains after 3.3 half lives? if that's the case, then (1/2)^3.3 = 10% A? This could so easily be E if I'm looking at it the other way around. Can someone please explain this?

5) It's asking for Maintenance Dose (CL is used). And Units should be a Rate. C

My only complaint is for the Units in the questions for 4 and 5. The Vd should be a VOLUME, right? I know its easy to convert given the patients weight, but its frustrating when Vd is given in a different way. Same goes with CL.

I'm still trying to figure out what 3 is asking.

So are these the kinds of questions I can expect for Pharmacokinetics? I hope so!
 The above post was thanked by: mulchandani161 (03-29-2013), nandish_m (03-29-2013)
#3
03-29-2013
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dint understand ques.3 though!

#4
03-30-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 112 Times in 75 Posts Reputation: 122
answers: e , a , e , a ( b ? ) , c

1) 400 mg of a drug is administered to a patient and the drug is later measured in
plasma to be 1 µg/ml. What is the apparent volume of distribution (Vd)?

e) 400 L

2) If a drug with a 2-hour half life is given with an initial dose of 8 mcg/ml, assuming
first-order kinetics, how much drug will be left at 6 hours?

a) 8 mcg/ml

3) What percentage of the steady-state drug concentration is achieved at 3.3 * t(1/2)?

e) 90% around 87%

4) A target concentration of 7.5 mg/L of theophylline is required for a 60 kg patient.
What is the loading dose, given the following: Vd = 0.5 L/kg, Cl = 0.04 L/kg/hr, t(1/2) =
9.3 hr?
a) 0.5 L/kg * 60 kg * 7.5 mg/L = 225 mg/h, infusion

b) 0.5 L/kg * 60 kg * 7.5 mg/L = 225 mg, bolus

5) A target concentration of 7.5 mg/L of theophylline is required for a 60 kg patient.
What is the steady state maintenance dose, given the following: Vd = 0.5 L/kg, Cl = 0.04
L/kg/hr, t(1/2) = 9.3 hr?

c) 0.04 L/kg/hr * 60 kg * 7.5 mg/L = 18 mg/h, infusion
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#5
03-30-2013
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 161 Times in 85 Posts Reputation: 171

Neha, mind explaining how you got 3? is it because they are actually infusing the drug over the course of 3.3 half lives? or am i completely missing the point?

as far as number 4, i'm pretty sure that loading dose is not a RATE. because VDd doesn't have time in its units. but CL does, which is why maintenence dose does turn out to be a rate (but not loading dose)

Also, what is your reasoning for question number 2? this is 1st order kinetics, which follows the characteristic half lives. if the half life is 2 hours. that means the concentration of drug gets halved every 2 hours.

so the first 2 hours = 4

next 2 hours = 2

final 2 hours = 1

and that's where you stop, because its asking for how much drug is left after 6 hours (2+2+2)
#6
03-30-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 112 Times in 75 Posts Reputation: 122

Quote:
 Originally Posted by ReggieMiller Neha, mind explaining how you got 3? is it because they are actually infusing the drug over the course of 3.3 half lives? or am i completely missing the point? as far as number 4, i'm pretty sure that loading dose is not a RATE. because VDd doesn't have time in its units. but CL does, which is why maintenence dose does turn out to be a rate (but not loading dose) Also, what is your reasoning for question number 2? this is 1st order kinetics, which follows the characteristic half lives. if the half life is 2 hours. that means the concentration of drug gets halved every 2 hours. so the first 2 hours = 4 next 2 hours = 2 final 2 hours = 1 and that's where you stop, because its asking for how much drug is left after 6 hours (2+2+2)
to reach the steady state we give the patient maintance dose in this way:
8mg becomes half around 4 mg at 1st t 1/2,
maintaince dose 8 mg + remaining 4mg =12 so
12 mg becomes 6mg at 2nd t 1/2
again maintaince dose 8mg + remaining 6mg = 14 mg
now 14mg becomes 7mg at 3rd t 1/2
so
now it concluded that at 3rd (3.3) t 1/2 life it should be around 7-8 mg.

anyway exactly i dont know the answer,i solved above question in that way.
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Last edited by neha_subh; 03-30-2013 at 04:14 AM.
#7
03-30-2013
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 161 Times in 85 Posts Reputation: 171

Quote:
 Originally Posted by neha_subh to reach the steady state we give the patient maintance dose in this way: loading dose 8mg and t 1/2 = 2hrs 8mg becomes half around 4 mg at 1st t 1/2, maintaince dose 8 mg + remaining 4mg =12 so 12 mg becomes 6mg at 2nd t 1/2 again maintaince dose 8mg + remaining 6mg = 14 mg now 14mg becomes 7mg at 3rd t 1/2 so now it concluded that at 3rd (3.3) t 1/2 life it should be around 7-8 mg. anyway exactly i dont know the answer,i solved above question in that way.
wait, which question is this an explanation for? number 2, or number 3?

You're explanation helps me understand question 3 now, because you are infusing, so you use that same basic principle. so the answer should be 90%, because that's how much drug you get after 3.3 half lives infusion. i think. still need clarification.

But for question 2 (which i think ur reffering to), the initial dose was 8mg. this was not infused over time, it was simply given at one time. and from that point on, we are seeing the concentration go down, according to half lives. so how can the concentration also be 8 after 3 half lives?
#8
03-30-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 112 Times in 75 Posts Reputation: 122

Quote:
 Originally Posted by ReggieMiller wait, which question is this an explanation for? number 2, or number 3? You're explanation helps me understand question 3 now, because you are infusing, so you use that same basic principle. so the answer should be 90%, because that's how much drug you get after 3.3 half lives infusion. i think. still need clarification. But for question 2 (which i think ur reffering to), the initial dose was 8mg. this was not infused over time, it was simply given at one time. and from that point on, we are seeing the concentration go down, according to half lives. so how can the concentration also be 8 after 3 half lives?
for question 3 i will say 90% (e).
for question 2 acording to first order kinetics mine answer wil be 1 ug/ml as you said its asking about first order kinetic not steady state or anything else.thanks
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#9
03-30-2013
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Hey everyone, some ppl have been asking me where I got these questions. I got them from the following site: http://pharmacologycorner.com/usmle-...ug-metabolism/
There you can get A LOT of pharm questions for free. There are even helpful videos. However I do want to state, that I don't think Step 1 will ask anything complicated. I was told to expect 1 calculation question and it should be on half-life, clearance, maintance dose or Loading dose. That's just my thinking. Best of luck!!!
 The above post was thanked by: Sawsan Shattel (03-30-2013)
#10
03-30-2013
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hi , do you have a site for physiolgy questions , a very good one ?
#11
03-30-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 112 Times in 75 Posts Reputation: 122

Quote:
 Originally Posted by ReggieMiller wait, which question is this an explanation for? number 2, or number 3? You're explanation helps me understand question 3 now, because you are infusing, so you use that same basic principle. so the answer should be 90%, because that's how much drug you get after 3.3 half lives infusion. i think. still need clarification. But for question 2 (which i think ur reffering to), the initial dose was 8mg. this was not infused over time, it was simply given at one time. and from that point on, we are seeing the concentration go down, according to half lives. so how can the concentration also be 8 after 3 half lives?
doctor reggiemiller pls find out the answer these question and please post here.omgstep has posted here website but stil i havnt found answers from that website,questions were there but not answers. thanks
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Last edited by neha_subh; 03-30-2013 at 08:17 PM.
#12
04-15-2013
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question 3: we know loading dose is equal to Vd * Cp (Cp=target conc.)
and Vd= 0.5 * 60= 30 L and Cp= 7.5 mg/l. putting in the valueswe get 225 mg,bolus. (b)

question 4: we know maintainence dose is equal to CL*Css (steady state conc i.e target conc.). and CL= 2.4 L/hr and Css =7.5 mg/l, putiing the values answer is (c)

IN BOTH THE ANSWERS EVEN THE UNITS MATCH
#13
05-12-2013
 USMLE Forums Newbie Steps History: Not yet Posts: 2 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10
Didnt have one not even a half life

Did not have a one !!

 Tags Pharmacology-, Step-1-Questions

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