Competitive antagonists MCQs With Answer

Competitive antagonists MCQs With Answer

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Introduction: This set of Competitive antagonists MCQs With Answer is designed for B.Pharm students to deepen understanding of competitive antagonist pharmacology and pharmacodynamics. It covers key concepts such as reversible/orthosteric blockade, surmountable antagonism, dose–response curve shifts, Schild plot analysis, pA2 and Ki estimation, and clinical implications of competitive inhibition. Questions emphasize mechanism, experimental interpretation, calculations, and examples (e.g., naloxone, propranolol), reinforcing receptor theory, affinity versus efficacy, and concentration–response relationships. Use these MCQs to test and strengthen your analytical skills in receptor pharmacology and therapeutic application. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the defining feature of a competitive antagonist at a receptor?

  • It activates the receptor to produce a maximal response.
  • It binds covalently to an allosteric site.
  • It reversibly competes with the agonist for the same binding site.
  • It reduces agonist efficacy without affecting potency.

Correct Answer: It reversibly competes with the agonist for the same binding site.

Q2. How does a reversible competitive antagonist typically affect an agonist concentration–response curve?

  • Decreases the maximal response (Emax).
  • Shifts the curve rightward without changing Emax (surmountable shift).
  • Shifts the curve leftward and increases potency.
  • Produces a nonparallel downward shift.

Correct Answer: Shifts the curve rightward without changing Emax (surmountable shift).

Q3. Which parameter quantifies antagonist potency in classical receptor pharmacology and is obtained from Schild analysis?

  • EC50
  • pA2
  • Intrinsic activity
  • Therapeutic index

Correct Answer: pA2

Q4. According to the Schild equation, the concentration ratio (CR) equals:

  • 1 + [Agonist]/EC50
  • 1 + [Antagonist]/KB
  • [Antagonist]/(1 + [Agonist])
  • KB / [Antagonist]

Correct Answer: 1 + [Antagonist]/KB

Q5. Which clinical example is a classic competitive antagonist used to reverse opioid overdose?

  • Flumazenil
  • Naloxone
  • Propranolol
  • Phenoxybenzamine

Correct Answer: Naloxone

Q6. In competitive antagonism, increasing the agonist concentration can:

  • Convert the antagonist into an agonist.
  • Overcome inhibition and restore maximal response.
  • Further decrease the maximal response permanently.
  • Change receptor subtype specificity.

Correct Answer: Overcome inhibition and restore maximal response.

Q7. Which statement correctly distinguishes affinity from efficacy?

  • Affinity is the ability to produce a response; efficacy is binding strength.
  • Affinity refers to binding strength; efficacy refers to the ability to activate the receptor.
  • Both affinity and efficacy mean the same in receptor theory.
  • Efficacy is only relevant for antagonists, not agonists.

Correct Answer: Affinity refers to binding strength; efficacy refers to the ability to activate the receptor.

Q8. What does a Schild plot slope near 1 indicate for an antagonist?

  • Noncompetitive irreversible antagonism.
  • Competitive reversible antagonism at a single receptor population.
  • Multiple receptor subtypes with mixed agonism.
  • Allosteric modulation with positive cooperativity.

Correct Answer: Competitive reversible antagonism at a single receptor population.

Q9. The equilibrium dissociation constant for an antagonist (KB) is best described as:

  • The concentration of antagonist producing 50% maximal response.
  • The concentration of antagonist that occupies 50% of receptors when agonist is absent.
  • The inverse of antagonist affinity (lower KB = higher affinity).
  • The rate constant for antagonist metabolism.

Correct Answer: The inverse of antagonist affinity (lower KB = higher affinity).

Q10. pA2 is defined as the negative logarithm of the antagonist concentration that:

  • Halves the maximal response of the agonist.
  • Doubles the agonist concentration required to produce the same response (i.e., CR = 2).
  • Completely blocks receptor binding.
  • Has no effect on agonist potency.

Correct Answer: Doubles the agonist concentration required to produce the same response (i.e., CR = 2).

Q11. Which experimental observation supports that an antagonist is competitive and reversible?

  • Increasing antagonist concentration reduces Emax.
  • Increasing agonist concentration restores Emax despite antagonist presence.
  • Antagonist causes irreversible receptor downregulation.
  • Antagonist produces time-dependent progressive blocking despite washout.

Correct Answer: Increasing agonist concentration restores Emax despite antagonist presence.

Q12. A drug that competes at the orthosteric site but binds slowly with long residence time is likely to behave as:

  • A purely reversible competitive antagonist with no clinical consequences.
  • An apparent insurmountable antagonist due to slow dissociation.
  • An agonist because slow binding increases efficacy.
  • An enzyme inhibitor unrelated to receptor interactions.

Correct Answer: An apparent insurmountable antagonist due to slow dissociation.

Q13. Which formula relates Ki to IC50 in simple competitive binding assays when substrate concentration equals Km?

  • Ki = IC50 / (1 + [S]/Km)
  • Ki = IC50 × (1 + [S]/Km)
  • Ki = IC50 + [S] – Km
  • Ki = 1 / (IC50 × Km)

Correct Answer: Ki = IC50 / (1 + [S]/Km)

Q14. Which of the following is true about a surmountable antagonism?

  • The agonist’s maximal effect is irreversibly lost.
  • The antagonism can be overcome by increasing agonist concentration.
  • The antagonist binds covalently to the receptor.
  • The phenomenon only occurs with non-receptor mediated drugs.

Correct Answer: The antagonism can be overcome by increasing agonist concentration.

Q15. In a Schild regression, the x-axis plots log[antagonist] and the y-axis plots:

  • log(Emax)
  • log(EC50)
  • log(CR – 1), where CR is the concentration ratio
  • percent receptor occupancy

Correct Answer: log(CR – 1), where CR is the concentration ratio

Q16. Which pharmacokinetic/pharmacodynamic factor can alter observed competitive antagonism in vivo?

  • Agonist intrinsic activity only
  • Antagonist absorption, distribution, and clearance altering free concentration
  • Agonist color
  • Receptor genetic code fixed and unchangeable

Correct Answer: Antagonist absorption, distribution, and clearance altering free concentration

Q17. Which drug is a competitive antagonist at β-adrenergic receptors commonly taught in B.Pharm?

  • Propranolol
  • Phenylephrine
  • Nitroprusside
  • Isoproterenol

Correct Answer: Propranolol

Q18. If a competitive antagonist has a KB of 10 nM, what is its approximate pKB (negative log of KB in molar units)?

  • 2
  • 8
  • 10
  • 9

Correct Answer: 8

Q19. A competitive antagonist selective for one receptor subtype will show which of the following?

  • Equal blockade of all receptor families regardless of sequence.
  • Preferential shift of agonist potency at that receptor subtype only.
  • Conversion of agonist efficacy into inverse agonism at other subtypes.
  • Permanent receptor elimination across tissues.

Correct Answer: Preferential shift of agonist potency at that receptor subtype only.

Q20. Which experimental manipulation would reduce the rightward shift produced by a competitive antagonist?

  • Increasing antagonist concentration
  • Decreasing agonist concentration
  • Increasing agonist concentration
  • Adding a noncompetitive antagonist

Correct Answer: Increasing agonist concentration

Q21. In receptor occupancy theory, the effect of a competitive antagonist is primarily explained by:

  • Altering receptor post-translational modification.
  • Decreasing the fraction of receptors occupied by agonist.
  • Increasing receptor synthesis rates.
  • Changing agonist metabolism in the synapse.

Correct Answer: Decreasing the fraction of receptors occupied by agonist.

Q22. Which describes a competitive antagonist acting at an allosteric (nonorthosteric) site?

  • It is an orthosteric competitive antagonist by definition.
  • Allosteric antagonists modulate agonist binding indirectly and are typically noncompetitive.
  • Allosteric binding always increases agonist potency.
  • Allosteric antagonists are identical to inverse agonists.

Correct Answer: Allosteric antagonists modulate agonist binding indirectly and are typically noncompetitive.

Q23. When calculating pA2 from Schild data, a higher pA2 value indicates:

  • Lower antagonist potency
  • Higher antagonist potency (greater affinity)
  • No relation to potency
  • Only reflects agonist efficacy

Correct Answer: Higher antagonist potency (greater affinity)

Q24. Which is a limitation of using pA2 as the sole measure of antagonist action?

  • pA2 gives information about antagonist intrinsic efficacy.
  • pA2 assumes competitive, reversible antagonism and a single receptor population.
  • pA2 measures pharmacokinetics rather than pharmacodynamics.
  • pA2 is applicable only to non-receptor mediated drugs.

Correct Answer: pA2 assumes competitive, reversible antagonism and a single receptor population.

Q25. In vivo, apparent non-surmountable antagonism may result from:

  • Rapid agonist absorption
  • Insufficient agonist concentration at the receptor due to clearance
  • Complete reversibility of antagonist binding
  • High intrinsic efficacy of the agonist

Correct Answer: Insufficient agonist concentration at the receptor due to clearance

Q26. Which property differentiates irreversible antagonists from classical reversible competitive antagonists?

  • Irreversible antagonists typically bind covalently and reduce Emax.
  • Irreversible antagonists are overcome by increasing agonist concentration.
  • Irreversible antagonists increase agonist potency but not affinity.
  • Irreversible antagonists always have a Schild slope of 1.

Correct Answer: Irreversible antagonists typically bind covalently and reduce Emax.

Q27. Which experimental result would indicate a competitive antagonist with multiple binding sites or nonuniform receptor populations?

  • Schild slope approximately equal to 1
  • Schild slope significantly different from 1
  • Complete restoration of Emax with low agonist doses
  • No change in EC50 with antagonist present

Correct Answer: Schild slope significantly different from 1

Q28. Which technique is commonly used to measure direct antagonist affinity at a receptor in vitro?

  • Radioligand binding assay to determine Ki
  • Hemoglobin electrophoresis
  • Complete blood count (CBC)
  • Electrocardiography (ECG)

Correct Answer: Radioligand binding assay to determine Ki

Q29. If an antagonist has fast on/off kinetics, clinical antagonism is likely to be:

  • Long-lasting despite drug clearance
  • Short-lived and closely related to plasma concentration
  • Independent of dose
  • Irreversible

Correct Answer: Short-lived and closely related to plasma concentration

Q30. Which statement best summarizes the therapeutic significance of competitive antagonists?

  • They permanently block receptors and are useful for one-time cures.
  • They provide dose-dependent reversible control of receptor-mediated effects and can be titrated or reversed by agonists when needed.
  • They exclusively act on enzymes rather than receptors.
  • They increase agonist intrinsic efficacy without binding receptors.

Correct Answer: They provide dose-dependent reversible control of receptor-mediated effects and can be titrated or reversed by agonists when needed.

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