Receptor theory and receptor quantitation MCQs With Answer

Introduction: This set of MCQs on Receptor Theory and Receptor Quantitation is designed for M.Pharm students aiming to deepen their understanding of drug–receptor interactions and quantitative receptor pharmacology. Questions cover core concepts such as affinity, efficacy, Kd, Bmax, receptor reserve, Scatchard and saturation binding, Schild analysis, competitive versus noncompetitive antagonism, partial agonism, radioligand assays, and interpretation of binding kinetics. Each item is crafted to test interpretation of experimental data, calculation-oriented reasoning, and mechanistic understanding required for advanced pharmacology coursework and exams. Use these MCQs to assess strengths, identify gaps, and practice applied problem solving in receptor pharmacology.

Q1. Which parameter most directly represents the equilibrium concentration of ligand at which half of the receptors are occupied in a saturation binding assay?

• dissociation constant (Kd)
• Bmax
• kon

Correct Answer: dissociation constant (Kd)

Q2. In a Scatchard plot of specific binding versus bound/free ligand, a linear plot with slope -1/Kd indicates:

• a single class of noninteracting receptors
• presence of multiple receptor subtypes with different affinities
• irreversible ligand binding
• positive cooperativity among binding sites

Correct Answer: a single class of noninteracting receptors

Q3. Bmax measured in a radioligand binding experiment corresponds to:

• the total number of binding sites available per unit protein or tissue
• the ligand concentration giving half-maximal response
• the rate constant for ligand association
• the fractional occupancy at EC50

Correct Answer: the total number of binding sites available per unit protein or tissue

Q4. The Cheng–Prusoff equation is used to convert an experimentally measured IC50 to:

• Ki, the equilibrium inhibition constant for a competitive antagonist
• Bmax of the receptor population
• the Hill coefficient
• the koff rate constant

Correct Answer: Ki, the equilibrium inhibition constant for a competitive antagonist

Q5. A partial agonist shows which of the following characteristics compared with a full agonist at the same receptor system?

• Lower maximal efficacy but may have similar or greater affinity
• Higher maximal efficacy and lower affinity
• Acts only as an antagonist in all systems
• Always has a lower binding affinity than a full agonist

Correct Answer: Lower maximal efficacy but may have similar or greater affinity

Q6. A Schild plot is primarily used to determine:

• whether antagonism is competitive and the pA2 value
• the Bmax in a binding assay
• the koff value from dissociation kinetics
• the Hill coefficient for cooperative binding

Correct Answer: whether antagonism is competitive and the pA2 value

Q7. In radioligand binding, an observed Hill coefficient significantly greater than 1 suggests:

• positive cooperativity or heterogeneous binding sites
• a pure single-site binding with no cooperativity
• that the ligand is a full agonist
• that the Scatchard plot will be linear

Correct Answer: positive cooperativity or heterogeneous binding sites

Q8. If kon = 1 x 10^7 M^-1 s^-1 and koff = 1 x 10^-2 s^-1, the Kd equals:

• 1 x 10^-9 M
• 1 x 10^5 M
• 1 x 10^-3 M
• 1 x 10^2 M

Correct Answer: 1 x 10^-9 M

Q9. Receptor reserve (spare receptors) is most directly inferred when:

• maximal physiological response occurs at less than full receptor occupancy
• a competitive antagonist produces a parallel rightward shift in dose–response without reducing Emax
• Kd is much larger than EC50
• agonist binding is irreversible

Correct Answer: maximal physiological response occurs at less than full receptor occupancy

Q10. A noncompetitive antagonist typically produces which change in an agonist concentration–response curve?

• Decrease in maximal response (Emax) with or without change in EC50
• Parallel rightward shift without change in Emax
• Leftward shift and increase in Emax
• No change in either EC50 or Emax

Correct Answer: Decrease in maximal response (Emax) with or without change in EC50

Q11. In a saturation binding assay, specific binding is calculated as:

• total binding minus nonspecific binding
• nonspecific binding divided by total binding
• total binding multiplied by free ligand concentration
• scatchard slope multiplied by Bmax

Correct Answer: total binding minus nonspecific binding

Q12. The Ki value obtained from a competitive binding assay depends on:

• the concentration and affinity of the radioligand used in the displacement experiment
• only the Bmax of the receptor preparation
• the maximum response produced by an agonist (Emax)
• the Hill coefficient exclusively

Correct Answer: the concentration and affinity of the radioligand used in the displacement experiment

Q13. Which of the following best describes an irreversible antagonist in binding assays?

• Forms a covalent bond or very slow-dissociating complex reducing available receptor number
• Competes reversibly with agonist at the same binding site producing parallel shifts
• Enhances agonist affinity without affecting efficacy
• Causes positive cooperativity between receptors

Correct Answer: Forms a covalent bond or very slow-dissociating complex reducing available receptor number

Q14. If a radioligand displacement curve yields an IC50 of 10 nM when radioligand concentration equals its Kd, the approximate Ki (using Cheng–Prusoff) is:

• 5 nM
• 10 nM
• 20 nM
• 100 nM

Correct Answer: 5 nM

Q15. Which experimental approach directly measures koff (the dissociation rate constant) for a ligand–receptor pair?

• Measure decrease in bound radioligand over time after rapid dilution or addition of excess unlabeled ligand
• Perform a Scatchard plot of equilibrium binding data
• Use Schild analysis with increasing antagonist concentrations
• Calculate IC50 in a competitive inhibition assay without kinetic monitoring

Correct Answer: Measure decrease in bound radioligand over time after rapid dilution or addition of excess unlabeled ligand

Q16. A system showing a receptor with two affinity states (high and low) in equilibrium commonly indicates:

• receptor coupling to G-proteins or other regulatory proteins producing different conformations
• that the ligand is non-specific and binding to many unrelated proteins
• that Bmax cannot be determined by saturation binding
• that the Hill coefficient must equal 1

Correct Answer: receptor coupling to G-proteins or other regulatory proteins producing different conformations

Q17. In vivo, the potency of a drug is often shifted relative to in vitro receptor affinity because of:

• pharmacokinetic factors, receptor reserve, and signal amplification in tissues
• changes in atomic mass of the drug molecule
• the inability of receptors to be quantified by binding assays
• the constancy of Bmax across tissues

Correct Answer: pharmacokinetic factors, receptor reserve, and signal amplification in tissues

Q18. When analyzing a displacement curve, a steep slope (Hill slope >1) most likely indicates:

• multiple binding sites or cooperative interactions
• a single noncooperative binding site
• complete competitive antagonism only
• that Kd equals Bmax

Correct Answer: multiple binding sites or cooperative interactions

Q19. Which statement about spare receptors is correct?

• Presence of spare receptors allows full response at submaximal receptor occupancy
• Spare receptors increase Kd for agonists
• Spare receptors are measured directly as a change in koff
• Systems with spare receptors cannot be antagonized by irreversible antagonists

Correct Answer: Presence of spare receptors allows full response at submaximal receptor occupancy

Q20. In a competitive antagonist experiment, doubling the antagonist concentration results in a 4-fold shift in agonist EC50. This is consistent with:

• antagonist acting with a Schild slope near 2 indicating non-ideal behavior or cooperative binding
• simple reversible competitive antagonism with Schild slope of 1
• irreversible antagonism reducing Bmax
• allosteric potentiation of the agonist

Correct Answer: antagonist acting with a Schild slope near 2 indicating non-ideal behavior or cooperative binding

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