








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




Please answers and explanations to this nbme Q
A 13yr old girl is brought to the emergency department by her father after he found her in semicomatose state with an empty bottle of antihistamine beside her bed. plasma diphenhydramine concentrations measured hourly for 6hr are shown.
Time plasma drug concentration (ng/ml) 1:00pm 240 2:00pm 202 3:00pm 170 4:00pm 143 5:00pm 120 6:00pm 101 Approximately how long after the first measurement(in hours)will take for this patient's plasma concentration to drop below 10ng/ml? a.10 b.12 c.16 d.20 e.24 f. 30 g.38 
#2




D  20? I'm not 100% so maybe someone can verify but here's my method:
To get to half the original concentration, it took from 1pm to 5pm = ie: 4hrs We need the concentration to drop below 10 so, our calculation is as follows: 240  120  60  30  15  7.5 ie each dash is 4 hours long assuming this is first order kinetics and since there are 5 dashes  5 X 4 = 20 hours... 
The above post was thanked by:  
achistikbenny (06232010), usmle100 (06232010) 
#3




i think the answer is d 20 hours
i solved it this way : at 5 pm, the concentration of the drug is half the original concentration, which means that the half life of this drug is from 1 pm to 5 pm which is 4 hours ... a 240 ng/ml needs 5 half lives to get below 10 ( 7.5 ng/dl exactly ) and this would be 5 half lives times 4 hours each = 20 hours ... i hope this solution will be correct and useful 
The above post was thanked by:  
achistikbenny (06232010) 


#4




me too got D. 20, thru same method as u

The above post was thanked by:  
achistikbenny (06232010) 
#5




i would go for B too...

The above post was thanked by:  
achistikbenny (06232010) 
#6





The above post was thanked by:  
achistikbenny (06232010) 
#7




it is d
its easy
T half is the time needed for plasma concentration of drug to decrease by half and her plasma conc of the drug was 240 at 1 pm and reach half at5 pm so t half = 4 hours after 1st t half 120 after 2nd thalf 60 after 3rd thalf 30 after 4th thalf 15 after 5th thalf 7.5 so it decrease below 10 ng after 5 thalf so after 20 hours so it is d for sure 
The above post was thanked by:  
achistikbenny (06232010) 
#8




hey sorry its D...may be i was too sleepy while replying.....

The above post was thanked by:  
achistikbenny (06232010) 
#9




thanks everybody for the explanations. D is the correct ans.

Tags 
Pharmacology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
pls help with nbme answers to form 5 and 6  achistikbenny  USMLE Step 1 Forum  0  06132010 11:39 PM 
Looking for NBME Answers  YoramFishke  General Topics Forum  1  11112009 10:42 PM 
NBME  somaind  USMLE Step 1 Forum  2  08122009 03:23 PM 
