WBR0902

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
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 an interaction similar to that of A and B?]]
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 maximal effect, a partial agonist only leads to less effect when compared to a full agonist. 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. Educational objective: Educational Objective: References: ]]
Approved	Approved::No
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