WBR0902

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
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::