








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




Three Pharmacokinetic Questions
1.What is the halftime of elimination for a drug that undergoes firstorder elimination with a rate constant of 0.1 minute?
A. 10 minutes B. 100 minutes C. 0.1 minutes D. 6.93 minutes E. 693 minutes 2.How many minutes are required for approximately 97% elimination of a drug undergoing firstorder elimination with a halftime of 10 minutes? A. 10 B. 30 C. 50 D. 70 E. 100 3.A new drug is being tested in a clinical trials. The PK properties of the drug reveals that the halflife of the drug is 6 hours. If a continuous iv. infusion were started on a research subject, how long would it take to reach 75% of steady state? A. 3 hours B. 6 hours C. 9 hours D. 12 hours E. 18 hours F. 24 hours 
#2




Ans.
Question 1. I choose D. 6.93 minutes, we have to remember that actually constant order for decay in first order kinetics is equal to 0.693, so solving for halflife would be t1/2=0.693/0.1.
Question 2. I choose C. 50 minutes. For 97% of elimination to occur, it is necessary for 5 halflives to pass, given that the half life of the drug is 10 minutes, you mujltiply it by 5. Question 3. I choose D. 12 hours, since it is in a constant infusion, to get to 75% of steady state we need 2 t1/2 to pass by, since the halflife is 6 hours, we multiply by 2. Great questions!!! 
The above post was thanked by:  
bebix (06112011), leo lakhani (06132011) 
#3




!......i wonder from where u get the constant first order decay ???
2 c 3 12 hrs 
#4




Constant order of decay
Quote:
So, to solve the problem, we use the formula for elimination constant, which is Ke. Ke=0.7(constant order for decay)/halflife, we clear the equation for halflife and we get the new equation: halflife=0.7/Ke. The question states that Ke is 0.1, and 0.7 is really 0.693 rounded up, so halflife=0.693/0.1 halflife=6.93 minutes 
The above post was thanked by:  
#5




Quote:

#6




In case anyone is wondering that 0.693 comes from ln2 (use a calculator).

#7




1. D
2. C 3. D Good question, had to look up the relationship between elimination constant and t1/2 again. Now I won't forget it! Thanks again bebix! 
The above post was thanked by:  
bebix (06122011) 
#8




1.What is the halftime of elimination for a drug that undergoes firstorder elimination with a rate constant of 0.1 minute?
t1/2=0.693/Ke t1/2=0.693/0.1 t1/2=6.93 D. 6.93 minutes 2.How many minutes are required for approximately 97% elimination of a drug undergoing firstorder elimination with a halftime of 10 minutes? 5half lives required to get appr. 97% so 10*5 =50 C. 50 3.A new drug is being tested in a clinical trials. The PK properties of the drug reveals that the halflife of the drug is 6 hours. If a continuous iv. infusion were started on a research subject, how long would it take to reach 75% of steady state? to get 75% of steady state we need 2 t1/2 as it's constant infu. so multiply the half life by 2 6*2=12 D. 12 hours 
The above post was thanked by:  
bebix (06122011) 
#10




1.D
2.c 3.D 
Tags 
Pharmacology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

