Mechanisms of drug action MCQs With Answer

Mechanisms of drug action MCQs With Answer offers B. Pharm students a focused, practical review of pharmacodynamics and molecular pharmacology. This resource covers receptor theory, agonists, antagonists, dose-response relationships, EC50, Emax, potency, efficacy, G-protein coupled receptors, ion channels, enzyme inhibition, allosteric modulation, and concepts like spare receptors, inverse agonism and biased signaling. Emphasis is on clinical and mechanistic examples, interpretation of Schild plots, Ki/IC50 relationships, and therapeutic index for safe dosing. Questions reinforce critical thinking needed for drug design, therapeutics, and exams. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which receptor class primarily acts through changes in intracellular second messengers such as cAMP or IP3?

• Ion channel-linked receptors
• G-protein coupled receptors
• Intracellular nuclear receptors
• Enzyme-linked receptors

Correct Answer: G-protein coupled receptors

Q2. According to occupancy theory, drug effect is directly proportional to which of the following?

• The number of drug molecules administered
• The number of receptors occupied by the drug
• The drug’s molecular weight
• The duration of drug therapy

Correct Answer: The number of receptors occupied by the drug

Q3. Intrinsic efficacy of an agonist refers to:

• The concentration required to bind 50% of receptors
• The maximum response a drug can produce when all receptors are occupied
• The degree to which a bound agonist activates the receptor to elicit a response
• The speed at which the drug is eliminated

Correct Answer: The degree to which a bound agonist activates the receptor to elicit a response

Q4. Which statement best distinguishes potency from efficacy?

• Potency is the maximum effect; efficacy is the dose required for effect
• Potency is the dose producing a given effect; efficacy is the maximum effect achievable
• Potency and efficacy mean the same thing
• Potency relates to safety, efficacy to absorption

Correct Answer: Potency is the dose producing a given effect; efficacy is the maximum effect achievable

Q5. EC50 is defined as the concentration of a drug that produces:

• 50% of the maximal possible effect
• 50% receptor occupancy only
• Half-life equal to 50 minutes
• Maximum toxic effect in 50% of subjects

Correct Answer: 50% of the maximal possible effect

Q6. What does Emax represent in a graded dose-response curve?

• The potency of the drug
• The drug concentration that produces 50% response
• The maximum achievable effect of the drug
• The time to reach peak effect

Correct Answer: The maximum achievable effect of the drug

Q7. Which feature is characteristic of a reversible competitive antagonist?

• Reduces Emax without changing EC50
• Causes a parallel rightward shift of the dose-response curve that is surmountable with agonist
• Binds covalently and irreversibly to receptors
• Increases intrinsic efficacy of the agonist

Correct Answer: Causes a parallel rightward shift of the dose-response curve that is surmountable with agonist

Q8. Noncompetitive antagonists typically cause which change in a dose-response curve?

• Parallel rightward shift without change in Emax
• Decrease in Emax, often without change in EC50
• Leftward shift indicating potentiation
• No effect on agonist response

Correct Answer: Decrease in Emax, often without change in EC50

Q9. A Schild plot slope close to unity suggests what about the antagonist?

• It is a noncompetitive antagonist
• It is a partial agonist
• It acts as a competitive antagonist at the receptor
• It has no effect on receptor binding

Correct Answer: It acts as a competitive antagonist at the receptor

Q10. When a partial agonist is given together with a full agonist, the partial agonist will:

• Always increase the maximal effect
• Act as an antagonist and reduce the overall effect
• Have no interaction with the full agonist
• Convert the full agonist to an inverse agonist

Correct Answer: Act as an antagonist and reduce the overall effect

Q11. Inverse agonists differ from antagonists because they:

• Bind only to activated receptors
• Stabilize receptors in an inactive state and decrease constitutive activity
• Increase receptor density on the cell surface
• Are more potent but less efficacious than agonists

Correct Answer: Stabilize receptors in an inactive state and decrease constitutive activity

Q12. Positive allosteric modulators (PAMs) enhance agonist responses by:

• Binding to the same site as the agonist and blocking it
• Increasing receptor expression at the gene level
• Binding to a distinct site and increasing affinity or efficacy of the agonist
• Causing irreversible receptor internalization

Correct Answer: Binding to a distinct site and increasing affinity or efficacy of the agonist

Q13. Presence of spare receptors means that:

• Maximum response can occur without all receptors being occupied
• All receptors must be occupied for any response
• Agonists cannot produce maximal effects
• Antagonists have no effect

Correct Answer: Maximum response can occur without all receptors being occupied

Q14. Quantal dose-response curves are used to determine:

• Individual graded responses over time
• Population-based frequency of a defined effect at different doses
• The exact receptor number per cell
• Drug chemical stability

Correct Answer: Population-based frequency of a defined effect at different doses

Q15. Therapeutic index (TI) is best described as:

• Ratio of toxic dose to effective dose indicating safety margin
• The absolute potency of a drug
• A measure of drug absorption
• Difference between EC50 and IC50

Correct Answer: Ratio of toxic dose to effective dose indicating safety margin

Q16. IC50 is the concentration of an inhibitor that:

• Produces 50% of maximum stimulatory effect
• Inhibits a biological process by 50% under specific assay conditions
• Occupies 50% of the receptors irreversibly
• Is equivalent to Ki for all inhibitors

Correct Answer: Inhibits a biological process by 50% under specific assay conditions

Q17. In classical enzyme kinetics, a competitive inhibitor produces which effect on Km and Vmax?

• Increases Km, Vmax unchanged
• Decreases Km, Vmax unchanged
• Vmax decreased, Km unchanged
• Both Km and Vmax decreased

Correct Answer: Increases Km, Vmax unchanged

Q18. Irreversible enzyme inhibitors typically cause:

• An increase in Emax of the substrate
• A reduction in the apparent number of active enzymes and decreased Vmax
• No change in enzyme activity
• Only transient inhibition that is easily reversible

Correct Answer: A reduction in the apparent number of active enzymes and decreased Vmax

Q19. Local anesthetics such as lidocaine produce anesthesia mainly by:

• Enhancing GABAergic inhibition centrally
• Blocking voltage-gated Na+ channels in a use-dependent manner
• Irreversibly blocking acetylcholine receptors
• Increasing membrane permeability to K+

Correct Answer: Blocking voltage-gated Na+ channels in a use-dependent manner

Q20. Activation of stimulatory Gs protein-coupled receptors typically increases intracellular levels of:

• cAMP
• IP3
• Calcium-binding proteins
• cGMP

Correct Answer: cAMP

Q21. Receptor desensitization often involves which of the following mechanisms?

• Immediate increase in receptor gene transcription
• Phosphorylation of receptor and arrestin-mediated internalization
• Permanent deletion of receptor genes
• Conversion of receptors into enzymes

Correct Answer: Phosphorylation of receptor and arrestin-mediated internalization

Q22. Tachyphylaxis refers to:

• A gradual increase in drug effect over months
• Rapid decrease in drug response after repeated administration over a short period
• A form of allergic reaction to drugs
• Enhanced potency after chronic exposure

Correct Answer: Rapid decrease in drug response after repeated administration over a short period

Q23. Which parameter directly represents drug efficacy in pharmacology?

• EC50 value
• Emax
• Ki value
• Volume of distribution

Correct Answer: Emax

Q24. Lower EC50 for drug A compared to drug B indicates that drug A is:

• Less potent than drug B
• More potent than drug B
• More efficacious than drug B
• More toxic than drug B

Correct Answer: More potent than drug B

Q25. pA2 determined from a Schild analysis provides information about:

• The intrinsic efficacy of an agonist
• The negative log of antagonist dissociation constant and competitive antagonism strength
• The metabolic half-life of a drug
• Drug absorption rate from the gut

Correct Answer: The negative log of antagonist dissociation constant and competitive antagonism strength

Q26. Which statement best differentiates pharmacokinetics from pharmacodynamics?

• Pharmacokinetics is what the drug does to the body; pharmacodynamics is what the body does to the drug
• Pharmacokinetics studies absorption, distribution, metabolism and excretion; pharmacodynamics studies drug effects and mechanisms
• They are interchangeable terms
• Pharmacodynamics measures only toxic effects

Correct Answer: Pharmacokinetics studies absorption, distribution, metabolism and excretion; pharmacodynamics studies drug effects and mechanisms

Q27. Ki is best described as:

• The concentration of inhibitor that gives 50% maximal activation
• The equilibrium dissociation constant for inhibitor binding, reflecting inhibitor affinity
• The toxic dose for 50% of animals
• The intrinsic activity of an agonist

Correct Answer: The equilibrium dissociation constant for inhibitor binding, reflecting inhibitor affinity

Q28. Aspirin irreversibly inhibits cyclooxygenase (COX) by:

• Reversibly blocking the active site non-covalently
• Acetylating a serine residue in the COX active site
• Degrading the COX mRNA
• Acting as a competitive antagonist at prostaglandin receptors

Correct Answer: Acetylating a serine residue in the COX active site

Q29. Receptor selectivity differs from receptor specificity in that selectivity means:

• A drug binds exclusively to one receptor subtype and no others
• A drug preferentially interacts with some receptor subtypes over others but can still affect multiple targets
• All drugs are non-selective by definition
• Selectivity only concerns pharmacokinetics

Correct Answer: A drug preferentially interacts with some receptor subtypes over others but can still affect multiple targets

Q30. Biased agonism (functional selectivity) describes a ligand that:

• Activates all downstream pathways equally
• Preferentially activates one intracellular signaling pathway over another via the same receptor
• Is always an inverse agonist
• Only binds to ion channels, not GPCRs

Correct Answer: Preferentially activates one intracellular signaling pathway over another via the same receptor

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com