The dose-response curve of acetylcholine (ACh) is shown in the graph. When an additional drug is introduced, the response to ACh increases (shifts upward). Which drug is most likely responsible for this effect?

1. A

   Physostigmine

2. B

   Bethanechol

3. C

   Adrenaline

4. D

   Atropine

Appeared in: INI-CET EXAM -2025

Explanation

• The question describes potentiation, where a drug increases the effect of another drug (ACh).
• Physostigmine is an acetylcholinesterase (AChE) inhibitor.
• By inhibiting the enzyme that degrades ACh, physostigmine increases the concentration and duration of action of ACh at the synapse.
• This increased availability of ACh leads to a greater physiological response for any given dose, represented by an upward shift in the dose-response curve.

Why Other Options Were Wrong

• Option B: Bethanechol is a direct-acting muscarinic agonist. It mimics the action of ACh but does not increase the effect of ACh itself. It would cause an additive effect, not potentiation.
• Option C: Adrenaline is an adrenergic agonist. It acts on a different receptor system (alpha and beta-adrenergic receptors) and often produces physiological effects that are opposite to those of acetylcholine.
• Option D: Atropine is a competitive antagonist at muscarinic receptors. It blocks the effects of ACh, causing the dose-response curve to shift to the right (requiring more ACh for the same effect), which is the opposite of what is described.

Related Visual

• Visual 1: Diagram: A graph showing different types of dose-response curve shifts. This would illustrate a normal curve, a rightward shift (antagonism, e.g., Atropine), and an upward shift (potentiation, e.g., Physostigmine).

Clinical Relevance

• Nursing practice connection: Use the key finding related to Pharmacodynamics of Cholinergic Drugs to guide bedside assessment, documentation, and the next nursing action.
• Understanding drug interactions is crucial for safe nursing practice. Potentiation can lead to exaggerated effects and toxicity.
• Acetylcholinesterase inhibitors like physostigmine and neostigmine are used to treat conditions like myasthenia gravis by increasing muscle strength.
• They are also used to reverse the effects of neuromuscular blocking agents after surgery.

How to Approach the Question

• First, analyze the graph. An 'upward shift' means that for the same dose of ACh, the effect is now greater. This is the definition of potentiation.
• Next, consider the mechanism of action of each drug option in relation to acetylcholine.
• Evaluate Physostigmine: It's an AChE inhibitor. By blocking the breakdown of ACh, it increases ACh levels, thus potentiating its effect. This matches the graph.
• Evaluate Bethanechol: It's a direct agonist. It acts like ACh but doesn't make ACh itself more powerful.
• Evaluate Adrenaline: It's an adrenergic agonist, acting on a different system, often with opposing effects.
• Evaluate Atropine: It's an antagonist. It would block ACh and shift the curve to the right, not up. Therefore, Physostigmine is the only option that causes potentiation.

Concept Tested & Keywords

• Concept Tested: Pharmacodynamics of Cholinergic Drugs
• Stem keywords: dose-response curve, acetylcholine (ACh), increases response, shifts upward
• Lead-in keywords: most likely
• Clinical cues: Upward shift of the dose-response curve indicates potentiation

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