WBR0902: Difference between revisions

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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor={{Rim}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Pharmacology
|MainCategory=Pharmacology
Line 21: Line 21:
|MainCategory=Pharmacology
|MainCategory=Pharmacology
|SubCategory=General Principles
|SubCategory=General Principles
|Prompt=A researcher is investigating the effects of two drugs A and B on blood pressure in rats.  The graph below shows the maximal effect of A in decreasing blood pressure when administered alone or when administered in combination with B.
|Prompt=A researcher is investigating the effects of two drugs A and B on blood pressure in rats.  The graph below shows the maximal effect of A in decreasing blood pressure when administered alone or when administered in combination with B. Which of the following combinations has similar pharmacodynamics curve to that of A and B combined?


[[File:Antagonist.PNG|450px]]
[[File:WBR0902.png|450px]]


Which of the following combinations has an interaction similar to that of A and B?
 
|Explanation=Efficacy is defined as the maximal effect a drug can produce. While a full agonist produces maximal effect, a partial agonist only leads to less effect when compared to a full agonist.  
|Explanation=Efficacy is defined as the maximal effect a drug can produce. While a full agonist produces a maximal effect, a partial agonist can only produce a fraction of that effect i.e. has a lower efficacy. The figure demonstrates that when A is administered alone, a maximal response is observed. However, when A and B are combined, the curve is shifted downwards signifying a decrease in efficacy. The dose response curve demonstrates that the response reaches a plateau and does not increase despite an increase in the dose of drug A. This is the classical effect of a non-competitive inhibitor causing an irreversible decrease in efficacy. Phenoxybenzamine is a non-competitive inhibitor of norepinephrine that fits the dose-response curve displayed.
Potency is defined effect is produced for a given concentration of a drug. The lower the concentration of a substance to produce an effect, the higher the affinity of the substance to the receptor and hence the higher the potency. According to the figure, when A is alone, a maximal effect is produced. However, when A and B are combined the curve is shifted downwards depicting decreased efficacy. An increase in the concentration of A was not able to overcome the effect of B on the efficacy. Hence B is a non-competitive inhibitor. Phenoxybenzamine is a noncompetitive inhibitor of norepinephrine.
|AnswerA=[[Naloxone]] and [[morphine]]
|AnswerA=[[Naloxone]] and [[morphine]]
|AnswerAExp=Naloxone is a competitive antagonist of morphine. When a competitive antagonist is present, the potency of the agonist is altered and the curve of the effect of the agonist by increasing dose will be shifted towards the right.
|AnswerAExp=Naloxone is a competitive antagonist of morphine. When a competitive antagonist is present, the potency of the agonist is altered and the curve of the effect of the agonist by increasing dose will be shifted towards the right.

Revision as of 20:25, 9 March 2015

 
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Serge Korjian)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Pharmacology
Sub Category SubCategory::General Principles
Prompt [[Prompt::A researcher is investigating the effects of two drugs A and B on blood pressure in rats. The graph below shows the maximal effect of A in decreasing blood pressure when administered alone or when administered in combination with B. Which of the following combinations has similar pharmacodynamics curve to that of A and B combined?

]]

Answer A [[AnswerA::Naloxone and morphine]]
Answer A Explanation AnswerAExp::Naloxone is a competitive antagonist of morphine. When a competitive antagonist is present, the potency of the agonist is altered and the curve of the effect of the agonist by increasing dose will be shifted towards the right.
Answer B [[AnswerB::Atropine and acetylcholine]]
Answer B Explanation AnswerBExp::Atropine is a competitive antagonist of acetylcholine. When a competitive antagonist is present, the potency of the agonist is altered and the curve of the effect of the agonist by increasing dose will be shifted towards the right.
Answer C [[AnswerC::Haloperidol and dopamine]]
Answer C Explanation AnswerCExp::haloperidol is a competitive antagonist of dopamine. When a competitive antagonist is present, the potency of the agonist is altered and the curve of the effect of the agonist by increasing dose will be shifted towards the right.
Answer D [[AnswerD::Ondansetron and serotonin]]
Answer D Explanation AnswerDExp::Serotonin is a competitive antagonist of ondansetron. When a competitive antagonist is present, the potency of the agonist is altered and the curve of the effect of the agonist by increasing dose will be shifted towards the right.
Answer E [[AnswerE::Noreprinephrine and phenoxybenzamine]]
Answer E Explanation AnswerEExp::Phenoxybenzamine is a noncompetitive inhibitor of norepinephrine. An increase in the concentration of norepinephrine is not able to overcome the effect of phenoxybenzamine .
Right Answer RightAnswer::E
Explanation [[Explanation::Efficacy is defined as the maximal effect a drug can produce. While a full agonist produces a maximal effect, a partial agonist can only produce a fraction of that effect i.e. has a lower efficacy. The figure demonstrates that when A is administered alone, a maximal response is observed. However, when A and B are combined, the curve is shifted downwards signifying a decrease in efficacy. The dose response curve demonstrates that the response reaches a plateau and does not increase despite an increase in the dose of drug A. This is the classical effect of a non-competitive inhibitor causing an irreversible decrease in efficacy. Phenoxybenzamine is a non-competitive inhibitor of norepinephrine that fits the dose-response curve displayed.

Educational Objective:
References: ]]

Approved Approved::Yes
Keyword WBRKeyword::Antagonist, WBRKeyword::competitive antagonist, WBRKeyword::non-competitive antagonist, WBRKeyword::phenoxybenzamine, WBRKeyword::Norepinephrine
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