Cholinergic Agonists — MCQs
A person was given a muscle relaxant that competitively blocks nicotinic receptors. Which of the following drugs is used for reversal of muscle relaxation after surgery?
All are organophosphorus poisons, except.
A patient comes to the casualty with organophosphate poisoning. He was started on atropine infusion and pralidoxime. After 2 hours, the patient had a sudden rise in temperature. What is the most likely cause of the fever?
Which medication is the primary treatment for muscarinic effects in acute organophosphate poisoning?
What is the primary use of Pirenzepine?
Mechanism of action of atropine in treatment of organophosphate poisoning is?
M2 cholinergic receptor is located at
A patient with diabetes and COPD developed postoperative urinary retention. Which of the following drugs can be used for short term treatment to relieve the symptoms of this person?
Which sympathomimetic drug is primarily known to increase heart rate?
Which of the following statements about clonidine is incorrect?
Cholinergic Agonists Indian Medical PG Practice Questions and MCQs
Question 1: A person was given a muscle relaxant that competitively blocks nicotinic receptors. Which of the following drugs is used for reversal of muscle relaxation after surgery?
- A. Carbachol
- B. Succinylcholine
- C. Physostigmine
- D. Neostigmine (Correct Answer)
Explanation: ***Neostigmine*** - **Neostigmine** is an **acetylcholinesterase inhibitor** that increases the amount of acetylcholine at the neuromuscular junction, thereby overcoming the competitive block at nicotinic receptors [1], [4]. - This increase in acetylcholine effectively reverses the paralysis caused by **nondepolarizing muscle relaxants**, making it useful for post-surgical recovery [2]. *Carbachol* - **Carbachol** is a **direct-acting cholinergic agonist** that stimulates both muscarinic and nicotinic receptors and is not typically used for reversing competitive neuromuscular blockade. - Its primary use is for glaucoma and to stimulate the bladder or bowels, not to counteract muscle relaxants. *Succinylcholine* - **Succinylcholine** is a **depolarizing muscle relaxant** and would prolong, rather than reverse, muscle blockade if administered after a competitive blocker [3]. - It works by initially causing depolarization and then preventing further muscle contraction, leading to paralysis. *Physostigmine* - **Physostigmine** is an **acetylcholinesterase inhibitor** that crosses the blood-brain barrier, making it more suitable for treating central anticholinergic toxicity rather than peripheral neuromuscular blockade. - While it inhibits acetylcholinesterase, its central effects and potential for seizures limit its use for reversing surgical muscle relaxation.
Question 2: All are organophosphorus poisons, except.
- A. Abate
- B. Dibenanone
- C. Propoxur (Correct Answer)
- D. Malathion
Explanation: ***Propoxur*** - **Propoxur** is a **carbamate insecticide**, not an organophosphorus compound. - Carbamates inhibit **acetylcholinesterase** reversibly, leading to similar cholinergic symptoms but with a generally shorter duration of action compared to organophosphates. - This is the primary answer as carbamates are the most commonly tested alternative to organophosphates. *Abate* - **Abate** (also known as **temephos**) is an **organophosphate insecticide**. - It is often used as a larvicide to control mosquito populations, particularly in water. - Contains phosphorus-based structure typical of organophosphate compounds. *Dibenanone* - **Dibenanone** is NOT a standard organophosphorus compound. - It is a **chlorinated hydrocarbon** or **organochlorine compound** used as an insecticide. - While this option is also technically not an organophosphate, **Propoxur (carbamate)** is the more classical answer as carbamates vs. organophosphates is a key distinction in toxicology. *Malathion* - **Malathion** is a well-known and widely used **organophosphate insecticide**. - It works by irreversibly inhibiting **acetylcholinesterase**, causing accumulation of acetylcholine at cholinergic synapses. - One of the most commonly encountered organophosphate compounds in forensic toxicology.
Question 3: A patient comes to the casualty with organophosphate poisoning. He was started on atropine infusion and pralidoxime. After 2 hours, the patient had a sudden rise in temperature. What is the most likely cause of the fever?
- A. Side effect of pralidoxime.
- B. Result of organophosphate poisoning.
- C. Unrelated or unknown cause.
- D. Fever due to atropine toxicity. (Correct Answer)
Explanation: ***Fever due to atropine toxicity.*** - **Atropine** blocks muscarinic receptors, leading to inhibition of **sweat glands** and subsequent rise in body temperature (hyperthermia), especially with high doses or prolonged infusion. - Given the patient is receiving an **atropine infusion** and developed fever, **atropine toxicity** is a primary concern. *Side effect of pralidoxime.* - While pralidoxime can cause side effects like dizziness, blurred vision, or tachycardia, **fever is not a typical side effect** of pralidoxime. - Pralidoxime works by **regenerating acetylcholinesterase** [2, 3] and does not directly interfere with thermoregulation in a way that would cause fever. *Result of organophosphate poisoning.* - **Organophosphate poisoning** typically causes **hypothermia** due to excessive cholinergic stimulation leading to peripheral vasodilation and increased sweating [1]. - **Fever** is not a direct result of the acute phase of organophosphate poisoning itself, but rather a complication of treatment or other factors. *Unrelated or unknown cause.* - While possible, it's less likely to be "unrelated or unknown" when a clear pharmacological explanation (**atropine toxicity**) exists for fever in the context of the patient's treatment. - It would be important to first rule out known causes related to the ongoing treatment before attributing it to an unknown cause.
Question 4: Which medication is the primary treatment for muscarinic effects in acute organophosphate poisoning?
- A. Atropine (Correct Answer)
- B. Tubocurarine
- C. Neostigmine
- D. Pralidoxime
Explanation: ***Atropine*** - **Atropine** is a **muscarinic receptor antagonist** that directly blocks the effects of excessive acetylcholine at muscarinic sites, thereby reversing symptoms like bradycardia, bronchospasm, and excessive secretions seen in organophosphate poisoning. - It is the **primary agent** used to manage the muscarinic symptoms and is titrated until bronchorrhea and bronchospasm resolve. *Neostigmine* - **Neostigmine** is an **acetylcholinesterase inhibitor**, which would worsen the condition by increasing acetylcholine levels further. - It is used in conditions like **myasthenia gravis** to improve muscle strength, not in organophosphate poisoning. *Tubocurarine* - **Tubocurarine** is a **nicotinic receptor antagonist**, specifically a competitive neuromuscular blocker. - While organophosphate poisoning can affect nicotinic receptors, tubocurarine is not the primary treatment for muscarinic effects and could worsen respiratory depression in this context. *Pralidoxime* - **Pralidoxime** (2-PAM) is an **acetylcholinesterase reactivator** that can regenerate the enzyme, thereby reversing both muscarinic and nicotinic effects. - While crucial for reversing nicotinic effects and preventing 'aging' of the enzyme, it is **not the primary treatment for acute muscarinic crisis**; atropine is.
Question 5: What is the primary use of Pirenzepine?
- A. Gastric ulcer (Correct Answer)
- B. Glaucoma
- C. Hypertension
- D. Congestive cardiac failure
Explanation: ***Gastric ulcer*** - **Pirenzepine** is a selective **M1 muscarinic antagonist** used to block acetylcholine's effects on gastric parietal cells. - By inhibiting M1 receptors, it **reduces gastric acid secretion**, thus facilitating the healing of gastric ulcers. *Glaucoma* - **Glaucoma** treatment primarily involves reducing intraocular pressure, often with **beta-blockers** or **prostaglandin analogs**. - Pirenzepine's mechanism of action is irrelevant to the **pathophysiology of glaucoma**. *Hypertension* - **Hypertension** is managed by various classes of drugs that target blood pressure regulation, such as **ACE inhibitors** or **calcium channel blockers**. - Pirenzepine does not have a direct role in the **management of blood pressure**. *Congestive cardiac failure* - **Congestive cardiac failure** treatment focuses on improving cardiac output and reducing fluid overload with drugs like **diuretics** or **ACE inhibitors**. - Pirenzepine has no therapeutic application in the **treatment of heart failure**.
Question 6: Mechanism of action of atropine in treatment of organophosphate poisoning is?
- A. It inhibits secretion of acetylcholine
- B. It has antimuscarinic activity (Correct Answer)
- C. It is reactivator of acetylcholine esterase enzyme
- D. It is agonist of acetylcholine receptors
Explanation: ***It has antimuscarinic activity*** - **Organophosphate poisoning** leads to **excessive acetylcholine** at muscarinic receptors, causing symptoms like miosis, bradycardia, and increased secretions. - **Atropine** is a **competitive antagonist** at these muscarinic receptors, thereby blocking the effects of excess acetylcholine. *It inhibits secretion of acetylcholine* - Atropine does not directly inhibit the secretion of **acetylcholine** from nerve terminals. - Its action is postsynaptic, specifically at the **receptor level**. *It is reactivator of acetylcholine esterase enzyme* - **Pralidoxime (2-PAM)** and other **oximes** are the drugs that reactivate **acetylcholinesterase**. - Atropine does not reactivate the enzyme; it only blocks the effects of acetylcholine. *It is agonist of acetylcholine receptors* - An **agonist** would mimic the effects of acetylcholine, which would worsen the symptoms of organophosphate poisoning. - Atropine is an **antagonist**, meaning it blocks the receptors.
Question 7: M2 cholinergic receptor is located at
- A. Glands
- B. Heart (Correct Answer)
- C. Lungs
- D. Skeletal muscle
Explanation: ***Heart*** - The **M2 muscarinic acetylcholine receptor** is predominantly located in the **heart**, where its activation mediates parasympathetic effects. - Activation of M2 receptors in the heart leads to a **decrease in heart rate** and force of contraction. *Glands* - **Glandular secretions**, such as saliva, tears, and sweat, are primarily mediated by **M3 muscarinic receptors**. - While M2 receptors may have some minor presence, M3 is the dominant subtype for secretory functions. *Lungs* - **Bronchoconstriction** and increased **mucus secretion** in the lungs are primarily mediated by **M3 muscarinic receptors**. - M2 receptors are present, but their role is often considered to be inhibitory to other M3 effects or to modulate neurotransmitter release. *Skeletal muscle* - **Skeletal muscle contraction** is mediated by **nicotinic acetylcholine receptors (Nm)** at the neuromuscular junction. - Muscarinic receptors, including M2, are not directly involved in skeletal muscle contraction.
Question 8: A patient with diabetes and COPD developed postoperative urinary retention. Which of the following drugs can be used for short term treatment to relieve the symptoms of this person?
- A. Bethanechol (Correct Answer)
- B. Methacholine
- C. Terazosin
- D. Tamsulosin
Explanation: ***Bethanechol*** - This **muscarinic agonist** increases detrusor muscle tone and bladder contraction, which helps to facilitate urination and relieve **postoperative urinary retention**. - It has a relatively selective action on the bladder with fewer serious systemic side effects compared to other cholinergic agonists, making it suitable for this scenario. - While bethanechol can potentially cause mild bronchoconstriction, its effects are **much less pronounced than methacholine**, and it remains the **standard treatment** for postoperative urinary retention even in patients with COPD when used cautiously and for short-term management. *Methacholine* - While also a **muscarinic agonist**, methacholine has a less selective action than bethanechol and more pronounced **cardiovascular and respiratory effects**. - It causes significant **bronchoconstriction** and is **contraindicated in COPD patients**. - Its primary clinical use is in the diagnosis of **bronchial hyperreactivity** (methacholine challenge test) and not for urinary retention. *Terazosin* - This is an **alpha-1 adrenergic antagonist** used to treat **benign prostatic hyperplasia (BPH)** and hypertension. - It works by relaxing smooth muscle in the prostate and bladder neck, which can improve urine flow in obstructive conditions but would **not stimulate bladder contraction** needed in **postoperative urinary retention** (which is a hypotonic/atonic bladder problem). *Tamsulosin* - Similar to terazosin, tamsulosin is a **selective alpha-1a adrenergic antagonist** primarily used for BPH. - While it relaxes smooth muscle in the prostate to improve urine flow in obstructive conditions, it does **not directly stimulate bladder muscle contraction** and, therefore, is not appropriate for initiating urination in **postoperative urinary retention**.
Question 9: Which sympathomimetic drug is primarily known to increase heart rate?
- A. Isoprenaline (Correct Answer)
- B. Phenylephrine
- C. Noradrenaline
- D. Adrenaline
Explanation: ***Isoprenaline*** - **Isoprenaline** (isoproterenol) is a non-selective beta-adrenergic agonist, with a strong affinity for **β1 and β2 receptors** [1]. - Its activation of **β1 receptors** in the heart leads to a significant increase in **heart rate (positive chronotropy)** and contractility (positive inotropy) [1]. - It is the **most potent chronotropic agent** among sympathomimetics and is primarily known for increasing heart rate [2]. *Phenylephrine* - **Phenylephrine** is a selective **α1 adrenergic agonist** that causes vasoconstriction [4]. - It increases blood pressure but typically causes **reflex bradycardia** (decreased heart rate) due to baroreceptor activation. - Does NOT directly increase heart rate. *Noradrenaline* - **Noradrenaline** (norepinephrine) primarily acts on **α1 receptors** causing vasoconstriction, and to a lesser extent on **β1 receptors** [3]. - While it can stimulate β1 receptors, its predominant effect is to increase **mean arterial pressure** through vasoconstriction, often causing **reflex bradycardia** [3]. *Adrenaline* - **Adrenaline** (epinephrine) acts on **α1, β1, and β2 receptors** [4]. While it does increase heart rate via **β1 receptor** stimulation, it also causes significant **vasoconstriction** (via α1) and **vasodilation** (via β2). - Its cardiovascular effects are more complex and dose-dependent compared to isoprenaline's specific chronotropic action.
Question 10: Which of the following statements about clonidine is incorrect?
- A. Alpha 2 receptor agonist
- B. Sudden withdrawal causes rebound hypertension
- C. Controls loose motions due to diabetic neuropathy
- D. First line for AMI (Correct Answer)
Explanation: ***First line for AMI*** - Clonidine is **not first-line** for **Acute Myocardial Infarction (AMI)** as it can cause **bradycardia** and **hypotension**, potentially worsening cardiac output. - First-line AMI treatments include **thrombolytics**, **antiplatelet agents** (aspirin), **beta-blockers**, and **ACE inhibitors** for optimal cardiac protection. *Alpha 2 receptor agonist* - Clonidine is indeed an **alpha-2 adrenergic receptor agonist** that acts centrally in the **medulla oblongata**. - It reduces **sympathetic outflow** from the CNS, leading to decreased **heart rate**, **blood pressure**, and **peripheral vascular resistance**. *Sudden withdrawal causes rebound hypertension* - Abrupt clonidine discontinuation causes dangerous **rebound hypertension** due to sudden loss of **sympathetic inhibition**. - **Gradual tapering** over 1-2 weeks is essential to prevent this potentially life-threatening complication. *Controls loose motions due to diabetic neuropathy* - Clonidine effectively treats **diabetic diarrhea** by stimulating **alpha-2 receptors** in the enteric nervous system. - It **slows intestinal transit** and **enhances fluid absorption**, making it useful for **autonomic neuropathy-related** gastrointestinal symptoms.
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