Medical Pharmacology Question Bank

Chapter 1: General Pharmacology — Module 3: Pharmacodynamics
Tier: Tier 2 — Conceptual Understanding

1. A researcher studies two drugs acting at the same receptor. Drug A produces a maximum response equal to 100% of the tissue maximum at an EC50 of 10 nM. Drug B produces a maximum response of 60% of the tissue maximum at an EC50 of 2 nM. Which of the following correctly characterizes the pharmacodynamic relationship between Drug A and Drug B?

A) Drug B is more efficacious than Drug A because it produces its maximum response at a lower concentration, indicating superior receptor activation capacity
B) Drug A is more potent than Drug B because it produces a higher maximum response, demonstrating greater intrinsic activity at the receptor
C) Drug B is more potent than Drug A (lower EC50), but Drug A has greater intrinsic efficacy (higher Emax); Drug B behaves as a partial agonist relative to Drug A
D) Drug A and Drug B have equivalent pharmacodynamic profiles because they act at the same receptor and therefore must share the same intrinsic efficacy
E) Drug B has greater affinity than Drug A because its EC50 is lower, and affinity and efficacy are equivalent properties at the same receptor

ANSWER: C

Rationale:

This question requires careful separation of potency and efficacy — two properties that are independent and frequently confused. Potency refers to the concentration of drug required to produce a given effect and is quantified by EC50; a lower EC50 indicates greater potency. Efficacy (in the pharmacodynamic sense of intrinsic efficacy or Emax) refers to the maximum response the drug can produce at saturating concentrations. Drug B has an EC50 of 2 nM compared to Drug A's 10 nM — Drug B is therefore five times more potent. However, Drug B's Emax is 60% of the tissue maximum, while Drug A achieves 100% — Drug A has greater intrinsic efficacy. Drug B, which cannot achieve the full tissue maximum regardless of dose, is by definition a partial agonist relative to Drug A (the full agonist). In the presence of Drug B at saturating concentrations, Drug A would be unable to increase the response above 60% — a hallmark of partial agonism. Option A incorrectly equates lower EC50 with greater efficacy — potency and efficacy are independent. Option B incorrectly equates higher Emax with greater potency — Emax reflects efficacy, not potency. Option D is incorrect — drugs acting at the same receptor can have entirely different intrinsic efficacies; receptor occupancy does not guarantee equal activation. Option E is incorrect — EC50 reflects the combined influence of affinity and efficacy, not affinity alone; furthermore, affinity (Kd) and efficacy are pharmacodynamically distinct properties.

2. Buprenorphine is used for opioid use disorder maintenance therapy and for analgesia. It is a partial agonist at the mu-opioid receptor with very high receptor affinity and slow receptor dissociation. Which of the following best explains the clinical consequence of buprenorphine's partial agonism when a patient maintained on buprenorphine requires additional opioid analgesia for acute pain?

A) Buprenorphine's partial agonism at mu receptors means it will synergistically enhance the analgesic effect of a full mu agonist such as morphine, because partial and full agonists acting at the same receptor always produce additive or supra-additive responses
B) Buprenorphine's high affinity and partial agonism mean it occupies a large proportion of mu receptors, producing a submaximal analgesic effect itself; if a full agonist such as morphine is added, buprenorphine competitively displaces morphine from mu receptors (due to its higher affinity) and limits the total achievable response to its own partial agonist ceiling — making acute pain management challenging and requiring specialized protocols
C) Buprenorphine's slow receptor dissociation means that full agonist opioids cannot bind to mu receptors at all in a buprenorphine-maintained patient, rendering all opioid analgesia completely ineffective regardless of dose
D) Buprenorphine acts exclusively as a kappa-opioid receptor antagonist in maintenance doses and has no interaction with mu-opioid receptor agonists used for analgesia
E) Partial agonists such as buprenorphine always have lower receptor affinity than full agonists; morphine will therefore readily displace buprenorphine from mu receptors and provide full analgesic effect at standard doses

ANSWER: B

Rationale:

Buprenorphine's clinical pharmacology in the context of acute pain superimposed on maintenance therapy illustrates the ceiling effect of partial agonism combined with competitive high-affinity receptor occupancy. Buprenorphine has an exceptionally high mu-opioid receptor affinity (Kd in the sub-nanomolar range) and a characteristically slow dissociation rate (t½ of receptor dissociation hours to days), meaning it occupies mu receptors tenaciously. As a partial agonist, it produces submaximal mu-receptor activation — providing some analgesia and opioid effect but below the maximum achievable with a full agonist. When a full agonist such as morphine is added, buprenorphine's high affinity competitively limits the proportion of receptors available to morphine. The combined response is constrained by buprenorphine's partial agonist ceiling — the total analgesic effect cannot exceed what buprenorphine's intrinsic efficacy permits, regardless of the morphine dose added. This creates a genuine clinical challenge: patients on buprenorphine maintenance experiencing acute pain (e.g., surgical pain, trauma) require specialized management, including continuing buprenorphine (to prevent withdrawal), using multimodal non-opioid analgesia (NSAIDs, ketamine, regional anesthesia), and potentially using very high doses of full agonists to overcome the competitive partial agonist occupancy — under close monitoring for respiratory depression if buprenorphine is partially displaced. Option A is incorrect — partial and full agonists acting at the same receptor do not produce synergistic responses; the partial agonist ceiling limits the combined maximum. Option C is incorrect — buprenorphine does not abolish all opioid analgesia; at very high doses of full agonists, partial displacement is achievable, though buprenorphine's high affinity makes this difficult. Option D is incorrect — buprenorphine's primary clinically relevant action in maintenance therapy is at mu-opioid receptors (partial agonism), not exclusively at kappa receptors. Option E is incorrect — buprenorphine has higher, not lower, mu-receptor affinity than morphine; this is precisely the pharmacological basis of its use in opioid use disorder (to block heroin/morphine binding) and of the clinical challenge described.

3. A new antipsychotic drug is described as having the following receptor binding profile: dopamine D2 receptor antagonism (Ki 0.8 nM), histamine H1 receptor antagonism (Ki 2.1 nM), muscarinic M1 receptor antagonism (Ki 4.5 nM), and alpha-1 adrenoceptor antagonism (Ki 6.2 nM). Based on this receptor binding profile alone, which of the following adverse effect predictions is most pharmacologically justified?

A) The drug will cause QT prolongation and torsades de pointes as its primary adverse effect, because all antipsychotics with dopamine D2 antagonism block cardiac hERG potassium channels
B) The drug is predicted to cause sedation (H1 antagonism), weight gain and metabolic effects (H1 antagonism), anticholinergic effects including dry mouth, constipation, urinary retention and cognitive impairment (M1 antagonism), and orthostatic hypotension (alpha-1 antagonism) — in addition to extrapyramidal effects and hyperprolactinemia from D2 blockade
C) The drug will cause only extrapyramidal side effects and hyperprolactinemia, because these are the only adverse effects attributable to antipsychotic drugs and all other receptor interactions are clinically negligible
D) The drug's alpha-1 antagonism will cause reflex tachycardia severe enough to require beta-blocker co-administration in all patients
E) The drug's M1 antagonism will enhance its antipsychotic efficacy by increasing dopaminergic tone in the mesolimbic pathway, reducing the dose required for D2 blockade

ANSWER: B

Rationale:

This question illustrates the fundamental principle that a drug's adverse effect profile is a direct pharmacological consequence of its receptor binding profile — every receptor it binds with meaningful affinity is a potential source of clinical effect, wanted or unwanted. The receptor binding data presented maps directly onto predictable adverse effects: D2 antagonism in the nigrostriatal pathway produces extrapyramidal side effects (EPS: parkinsonism, akathisia, dystonia, tardive dyskinesia) and in the tuberoinfundibular pathway produces hyperprolactinemia (galactorrhea, amenorrhea, sexual dysfunction). H1 antagonism produces sedation (via central H1 blockade) and contributes to weight gain and metabolic dysregulation — the Ki of 2.1 nM for H1 indicates potent histamine blockade. M1 muscarinic antagonism produces the classic anticholinergic triad of dry mouth, constipation, urinary retention, blurred vision, and cognitive impairment (particularly in elderly patients). Alpha-1 adrenoceptor antagonism produces vasodilation and orthostatic hypotension, with reflex tachycardia in some patients. This receptor binding profile is consistent with a low-potency typical or atypical antipsychotic (resembling chlorpromazine or clozapine). Option A is incorrect — QT prolongation is associated with hERG channel blockade, which is a separate property not encoded in the D2 binding affinity; not all D2 antagonists block hERG. Option C is incorrect — the multi-receptor binding profile predicts a broad adverse effect spectrum well beyond EPS and hyperprolactinemia. Option D is incorrect — alpha-1 antagonism causes orthostatic hypotension and may produce reflex tachycardia, but this is not universally severe enough to require beta-blocker co-administration in all patients; it is a monitoring concern, not an invariable therapeutic requirement. Option E is incorrect — M1 antagonism does not enhance antipsychotic efficacy through dopaminergic mechanisms; anticholinergic effects on cognition are adverse, not therapeutic in this context.

4. The concept of spare receptors (receptor reserve) has important implications for the pharmacodynamic effects of irreversible antagonists. Which of the following best explains the clinical significance of receptor reserve in the context of irreversible receptor blockade?

A) Spare receptors allow a drug to produce its maximum response even when receptor reserve is fully occupied, because spare receptors are activated independently of the main receptor population by a separate signaling pathway
B) In tissues with a large receptor reserve, a substantial proportion of receptors can be irreversibly blocked without any reduction in Emax, because the remaining unblocked receptors are sufficient to generate the full tissue maximum response when fully occupied by agonist; however, the EC50 of the agonist increases (potency decreases) as the receptor reserve is progressively depleted
C) Receptor reserve has no pharmacodynamic significance because all receptors in a tissue must be occupied simultaneously to produce any measurable biological response
D) In tissues with receptor reserve, irreversible antagonists shift the agonist dose-response curve to the right without reducing Emax — the same pharmacodynamic effect as a competitive reversible antagonist — making the two types of antagonism pharmacodynamically indistinguishable
E) Spare receptors compensate for irreversible receptor blockade by upregulating receptor gene transcription within minutes, restoring total receptor number and Emax before any clinical effect of the antagonist is observed

ANSWER: B

Rationale:

Receptor reserve (spare receptors) refers to the phenomenon, first described by Stephenson, in which Emax can be achieved at agonist concentrations that occupy far less than 100% of the total receptor population. In a tissue where only 10% receptor occupancy is required for Emax, the remaining 90% of receptors are "spare" — not required for the maximal response under normal conditions. The existence of spare receptors has a profound and clinically important implication for irreversible antagonists: if an irreversible antagonist blocks, say, 50% of receptors in a tissue with 90% spare receptors, the remaining 50% of receptors still far exceeds the 10% needed for Emax — the maximum response is therefore fully preserved, and the only observable effect is a rightward shift of the agonist dose-response curve (reduced potency, higher EC50 required). Only when the irreversible antagonist has blocked enough receptors to reduce the available pool below the minimum required for Emax will the maximum response begin to fall. This explains why phenoxybenzamine (irreversible alpha-blocker) at low doses shifts the norepinephrine dose-response curve rightward without reducing the maximum pressor response — and why progressively higher doses eventually reduce Emax as the receptor reserve is exhausted. Option A is incorrect — spare receptors are not a separate receptor population with independent signaling; they are receptors of the same type that are not required for Emax under normal conditions. Option C is incorrect — the existence of spare receptors demonstrates that simultaneous occupancy of all receptors is not required for Emax. Option D is incorrect — this is the key distinction: at low doses of irreversible antagonist (when receptor reserve absorbs the blockade), the curve shifts right without reducing Emax, mimicking competitive antagonism; but at higher doses that exhaust receptor reserve, Emax falls — an effect that cannot be overcome by increasing agonist concentration, unlike competitive antagonism. Option E is incorrect — receptor gene transcription and upregulation occur on a timescale of hours to days, not minutes; no compensatory upregulation occurs within the timeframe of acute irreversible blockade.

5. Aripiprazole is classified as a dopamine D2 receptor partial agonist and is used as an antipsychotic. Which of the following best explains the theoretical pharmacodynamic rationale for using a partial agonist rather than a full antagonist at D2 receptors in schizophrenia?

A) Partial agonism at D2 receptors provides full receptor blockade in the mesolimbic pathway (treating positive symptoms) while simultaneously activating D2 receptors in the nigrostriatal pathway (preventing extrapyramidal side effects) — a binary switching mechanism based on endogenous dopamine concentration
B) In brain regions where dopamine is pathologically elevated (mesolimbic pathway in schizophrenia), aripiprazole's partial agonism functionally reduces dopaminergic transmission by competing with dopamine and replacing full agonist activation with submaximal partial agonist activation; in regions where dopamine is deficient (mesocortical pathway), aripiprazole provides partial agonist stimulation above the deficient baseline — theoretically addressing both positive and negative/cognitive symptoms
C) Aripiprazole's partial agonism at D2 receptors produces stronger antipsychotic efficacy than full D2 antagonists because partial agonists always produce greater receptor activation than full agonists at therapeutic concentrations
D) As a D2 partial agonist, aripiprazole avoids all dopamine-related adverse effects including hyperprolactinemia and extrapyramidal side effects completely, because partial agonists never produce adverse effects at their primary receptor target
E) Aripiprazole's partial agonism stabilizes the D2 receptor in the inactive conformation more effectively than full antagonists, making it pharmacodynamically equivalent to an irreversible antagonist at therapeutic doses

ANSWER: B

Rationale:

The dopamine hypothesis of schizophrenia posits that positive symptoms (hallucinations, delusions) arise from pathological hyperdopaminergia in the mesolimbic pathway, while negative symptoms and cognitive dysfunction may reflect relative hypodopaminergia in the mesocortical pathway. Full D2 antagonists address mesolimbic hyperdopaminergia effectively but simultaneously block D2 receptors in the nigrostriatal pathway (causing EPS) and tuberoinfundibular pathway (causing hyperprolactinemia), and may worsen mesocortical hypodopaminergia (worsening negative and cognitive symptoms). The rationale for a partial agonist such as aripiprazole is that its intrinsic efficacy at D2 receptors is intermediate between zero (full antagonist) and 1.0 (full agonist). In mesolimbic regions where dopamine is pathologically elevated, aripiprazole competes with dopamine at D2 receptors and, by replacing full dopamine activation with partial agonist submaximal activation, functionally reduces net D2 receptor signaling — an effective "functional antagonism." In mesocortical regions where dopamine is deficient, aripiprazole's partial agonism provides some D2 stimulation above the inadequate baseline — potentially ameliorating negative and cognitive symptoms. In the nigrostriatal pathway, the partial agonism may be less likely to produce EPS than full blockade. This dopamine system stabilizer concept is the theoretical framework for aripiprazole and other partial agonists in psychiatry. Option A is incorrect — the mechanism is not a binary switch but a graded, concentration-dependent functional antagonism in high-dopamine regions and partial agonism in low-dopamine regions; aripiprazole does not fully block D2 receptors in any pathway. Option C is incorrect — partial agonists produce submaximal, not greater, receptor activation compared to full agonists; their advantage is functional regulation, not superior activation. Option D is incorrect — aripiprazole does cause some EPS and akathisia, and can cause mild hyperprolactinemia reduction (or occasionally prolactin elevation depending on context); it does not eliminate all dopamine-related adverse effects. Option E is incorrect — partial agonists do not stabilize receptors in the inactive conformation more effectively than full antagonists; an inverse agonist or full antagonist would more effectively stabilize the inactive state.