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#1
06-11-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269
Three Pharmacokinetic Questions

1.-What is the half-time of elimination for a drug that undergoes first-order 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 first-order elimination with a half-time 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 half-life 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
 The above post was thanked by: 7802 (06-12-2011), Claus_CU (06-12-2011), drnirajmavani (06-13-2011), elmus (06-12-2011), NGaleas (06-11-2011), rani yako (06-12-2011), rulz (06-12-2011), struggle (06-11-2011), usluipek (06-11-2011)

#2
06-11-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 124 Threads: 5 Thanked 204 Times in 70 Posts Reputation: 214
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 (06-11-2011), leo lakhani (06-13-2011)
#3
06-11-2011
 USMLE Forums Scout Steps History: Step 1 Only Posts: 35 Threads: 1 Thanked 49 Times in 23 Posts Reputation: 59

!-......i wonder from where u get the constant first order decay ???
2- c
3- 12 hrs
#4
06-11-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 124 Threads: 5 Thanked 204 Times in 70 Posts Reputation: 214
Constant order of decay

Quote:
 Originally Posted by Al-Saoudi !-......i wonder from where u get the constant first order decay ??? 2- c 3- 12 hrs
The constant order for decay in first order kinetics is the "0.7" value we find in the clearance equations: Cl=Vd x 0.7/halflife.
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: 1TA2B (06-13-2011), Al-Saoudi (06-11-2011), drnirajmavani (06-13-2011)
#5
06-11-2011
 USMLE Forums Scout Steps History: Step 1 Only Posts: 35 Threads: 1 Thanked 49 Times in 23 Posts Reputation: 59

Quote:
 Originally Posted by NGaleas The constant order for decay in first order kinetics is the "0.7" value we find in the clearance equations: Cl=Vd x 0.7/halflife. 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
tnx alot
#6
06-12-2011
 USMLE Forums Scout Steps History: Not yet Posts: 46 Threads: 3 Thanked 34 Times in 20 Posts Reputation: 44

In case anyone is wondering that 0.693 comes from ln2 (use a calculator).
#7
06-12-2011
 USMLE Forums Guru Steps History: Not yet Posts: 298 Threads: 45 Thanked 713 Times in 210 Posts Reputation: 723

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 (06-12-2011)
#8
06-12-2011
 USMLE Forums Newbie Steps History: --- Posts: 4 Threads: 1 Thanked 1 Time in 1 Post Reputation: 11

1.-What is the half-time of elimination for a drug that undergoes first-order 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 first-order elimination with a half-time 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 half-life 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 (06-12-2011)
#9
06-12-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 88 Times in 61 Posts Reputation: 98

d, c, and d

good one!
 The above post was thanked by: bebix (06-12-2011)
#10
06-13-2011
 USMLE Forums Scout Steps History: Step 1 Only Posts: 76 Threads: 3 Thanked 48 Times in 35 Posts Reputation: 58

1.D
2.c
3.D

 Tags Pharmacology-, Step-1-Questions

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