








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#1




Challenging question: How long the drug would be effective?
A 400 mg dose of a drug has therapeutic efficacy for 8 h. If the clearance of the drug is 2.52 L/h and the drug is distributed into total body water, for how long would a 800 mg dose be effective in a patient weighing 60 kg?
A) 8 h B) 10 h C) 16 h D) 18 h E) 24 h F) 28 h Last edited by bebix; 06152011 at 08:06 AM. Reason: typo, ty pass7! 
The above post was thanked by:  
AlSaoudi (06142011), Claus_CU (06142011), nevillenic (06152011), pass7 (06142011), struggle (06152011) 
#2




Yikes, no idea!
I would guess randomly D because for some reason I feel like it would be efficacious for a bit longer than 2x the original. Either that or B Last edited by apx85; 06142011 at 02:45 PM. 
The above post was thanked by:  
bebix (06142011) 
#3




i think d answer iz 10h.. option b!!
as far as i think d drug follows a zero order kinetics!! Last edited by sidra; 06142011 at 01:15 PM. Reason: i thought again!!! 
The above post was thanked by:  
bebix (06142011) 
#4




why zero order kinetics???

#7




@bebix...where did u find this qs???

#9




correct answer
Well, the correct answer is D) 18 h
First, we need to calculate the half time of this drug: general equation: t1/2 = 0.7 / ke, where ke = Cl / VD In this case VD = 60% of 60 Kg = Total body water = 36 L ke = 2.52 / 36 = 0.07 then, t1/2 = 0.7 / 0.07 = 10 h We know that "400 mg dose of a drug has therapeutic efficacy for 8 h" [which is NOT the same as t1/2] So, if we use 800mg => 1 t1/2 later (10 hr later) => 400 mg => and this 400mg still has a therapeutic efficacy for 8 h. for how long would a 800 mg dose be effective in this patient then? 10h + 8h = 18h 
#11




but it doesn't say whether it's firstorderelimination or zeroorder elimination???????????
but it doesn't say whether it's firstorderelimination or zeroorder elimination??????????? how about zero order elimination,?
Quote:

The above post was thanked by:  
bebix (06152011) 
#12




Quote:
i mean this is wat i got from the calculation .. t1/2= 0.7 x3600/2.52 =1000mints =16.6hrs so 800mg =16+8hrs=24hrs ????? wat do u think?? 
The above post was thanked by:  
bebix (06152011) 
#13




Quote:
@pass7 = you are absolutly right...the clearence should be 2.52L/h Last edited by bebix; 06152011 at 08:06 AM. 
The above post was thanked by:  
pass7 (06162011) 
#14




actually by changing ml to L and min to hours we are getting wrong calculations or may be i am weak in maths,,

#15




Quote:
the whole point here is to know the formulas...please use Cl in L/h...if your answer is 18h, your math is OK 
The above post was thanked by:  
struggle (06152011) 
#16




sheesh.....I just followed a quick and easy method
Vd = 60 x .6 = 36 CL = given as 2.. 1T(1/2) = .7(Vd/CL) = 10 800mg after 1 half life = 400 mg (takes 10 hrs) 400mg = efficacious for 8hrs 10+8 = 18 
Tags 
Pharmacology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Is this drug effective for heartburn!  bebix  USMLE Step 1 Forum  7  08092011 01:53 AM 
Effective Way to do the USMLE World question bank  donofitaly  USMLE Step 1 Forum  4  02052011 08:13 AM 
question  lithium toxicity drugdrug interaction  mtoi  USMLE Step 1 Forum  5  10312010 12:48 PM 
Challenging Question #9  MedicalExams  USMLE Step 2 CK Forum  5  10172010 02:07 PM 
Challenging Question #10  MedicalExams  USMLE Step 2 CK Forum  2  09162010 11:13 AM 
