Pharmacodynamics Quiz

Focused SBA-style practice, followed by the answer and explanation for every question.

Free now: 5 questions
Full bank: 7 questions
Answers: Explained

Free question

Try one with the answer

A question from this quiz. Answer it to see the explanation students get.

Which statement best describes drug affinity in pharmacodynamics?

Affinity is measured by the concentration needed to produce half of the maximal response, because that concentration shows how tightly the drug binds. An agonist with high affinity produces a small response, because tight binding holds the receptor in the inactive state. A drug with high affinity also has high efficacy, because tight binding produces a large biological response. A low dissociation constant (Kd) indicates high affinity, because less drug is needed to occupy half of the receptors. A high dissociation constant (Kd) indicates high intrinsic activity, because a drug that dissociates readily activates the receptor more strongly.

A Affinity is measured by the concentration needed to produce half of the maximal response, because that concentration shows how tightly the drug binds.
B An agonist with high affinity produces a small response, because tight binding holds the receptor in the inactive state.
C A drug with high affinity also has high efficacy, because tight binding produces a large biological response.
D A low dissociation constant (Kd) indicates high affinity, because less drug is needed to occupy half of the receptors. ✓
E A high dissociation constant (Kd) indicates high intrinsic activity, because a drug that dissociates readily activates the receptor more strongly.

The answer is D: A low dissociation constant (Kd) indicates high affinity, because less drug is needed to occupy half of the receptors.

Explanation

Why D is correct

See Affinity. Option D is correct. Binding a target has two separate properties: affinity, which is how readily and tightly a drug binds to its receptor, and efficacy, which is what happens once it has bound. Affinity is measured by the dissociation constant, Kd, the concentration of drug at which half of the available receptors are occupied at equilibrium. A low Kd means that only a little drug is needed to occupy half the receptors, so the drug binds tightly and has a high affinity. A high Kd means loose binding and a low affinity. Affinity is purely a measurement of binding and not of the biological response: an antagonist can bind with a very high affinity and still cause no effect by itself. It is worth keeping Kd (binding) separate from the EC50 (the concentration that gives half of the maximum response), because a response can depend on more than binding. As a simple way to separate the two ideas, affinity asks whether the drug binds, and efficacy asks whether it switches the receptor on once it is bound.

Affinity: how readily and tightly a drug binds to its receptor.
Dissociation constant: Kd is the drug concentration at which half of the receptors are occupied at equilibrium.
Low Kd: only a little drug is needed to occupy half the receptors, so binding is tight and affinity is high.
Binding only: affinity measures binding, not the biological response.

Why the other options are incorrect

A. Affinity is measured by the concentration needed to produce half of the maximal response, because that concentration shows how tightly the drug binds.: The concentration producing half of the maximal response is the EC50, a measure of potency, not affinity. Affinity is measured by the Kd, the concentration at which half of the receptors are occupied.
B. An agonist with high affinity produces a small response, because tight binding holds the receptor in the inactive state.: High affinity does not limit the size of the response; a full agonist can bind tightly and still produce the maximum response. The size of the response is determined by efficacy, not by how tightly the drug binds.
C. A drug with high affinity also has high efficacy, because tight binding produces a large biological response.: Affinity and efficacy are separate properties; a drug can bind tightly but fail to activate the receptor, as an antagonist does. Tight binding alone does not produce a response, so high affinity does not guarantee high efficacy.
E. A high dissociation constant (Kd) indicates high intrinsic activity, because a drug that dissociates readily activates the receptor more strongly.: A high Kd means loose binding and low affinity; it does not indicate intrinsic activity, which is efficacy. Dissociating readily from the receptor does not activate it, so this option confuses binding with the response after binding.

Easy

With Premium, questions you miss become Revision Coach flashcards, scheduled for review.

Question preview

What you will practise

More questions from this set. Start the quiz to answer them and see each explanation.

1
Which statement best describes the pharmacological behaviour of a partial agonist at a receptor?
2
Which statement best describes the effect of a reversible competitive antagonist on an agonist dose-response curve at equilibrium?

Need wider coverage?

Explore every Pharmacology topic set, or build a tailored mixed session.

Browse subject quizzes