Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 581: A 30-year-old male is diagnosed with myasthenia gravis. Which of the following drugs is commonly used for the symptomatic treatment of myasthenia gravis?

A. Pyridostigmine (Correct Answer)
B. Atropine
C. Cyclobenzaprine
D. Diazepam

Explanation: ***Pyridostigmine*** - **Pyridostigmine** is an **acetylcholinesterase inhibitor** that increases the amount of acetylcholine available at the neuromuscular junction, improving muscle strength. - It is considered a cornerstone for **symptomatic treatment** of myasthenia gravis, providing temporary relief from weakness. *Atropine* - **Atropine** is a **muscarinic acetylcholine receptor antagonist** and would worsen myasthenia gravis by blocking acetylcholine's effects at muscarinic receptors. - It is sometimes used to counteract the **side effects** of acetylcholinesterase inhibitors (e.g., bradycardia, increased secretions), but not for the primary treatment of myasthenia gravis. *Cyclobenzaprine* - **Cyclobenzaprine** is a **skeletal muscle relaxant** used to treat muscle spasms, not the weakness associated with autoimmune conditions like myasthenia gravis. - Its mechanism involves effects on the central nervous system, and it has no direct role in improving **neuromuscular transmission** in myasthenia gravis. *Diazepam* - **Diazepam** is a **benzodiazepine** primarily used for anxiety, seizures, and muscle spasms, acting as a CNS depressant. - It does not address the underlying **neuromuscular junction defect** in myasthenia gravis and can even worsen muscle weakness due to its sedative effects.

Question 582: Which of the following statements about the action of anticholinergic drugs is false?

A. Atropine is a CNS stimulant
B. Atropine causes bronchoconstriction (Correct Answer)
C. Atropine can increase the chances of hyperthermia in children
D. Atropine causes mydriasis, abolition of light reflex and cycloplegia

Explanation: ***Atropine causes bronchoconstriction*** - Atropine is an **anticholinergic drug** that blocks muscarinic receptors (M3) in the bronchi, leading to **bronchodilation**, not bronchoconstriction. - Bronchoconstriction is typically mediated by **acetylcholine** acting on M3 receptors, which atropine inhibits. *Atropine is a CNS stimulant* - While atropine primarily acts peripherally, higher doses can cross the **blood-brain barrier** and cause central nervous system effects, including initial stimulation followed by depression in severe overdose. - Initial stimulation can manifest as **restlessness**, **agitation**, and even hallucinations. *Atropine can increase the chances of hyperthermia in children* - Atropine inhibits **sweat gland activity** by blocking muscarinic receptors (M3), which can impair heat dissipation. - This effect, particularly in children who have a less developed thermoregulatory system, can lead to **atropine fever** or hyperthermia. *Atropine causes mydriasis, abolition of light reflex and cycloplegia* - Atropine blocks **muscarinic receptors** (M3) in the iris sphincter muscle, causing **mydriasis** (pupil dilation). - It also paralyzes the ciliary muscle (M3 receptor blockade), leading to **cycloplegia** (loss of accommodation) and abolition of the light reflex.

Question 583: Oxybutynin acts by which mechanism?

A. Adrenergic receptor antagonist
B. Histaminic antagonist
C. Serotonergic antagonist
D. Muscarinic receptor antagonist (Correct Answer)

Explanation: ***Muscarinic receptor antagonist*** - **Oxybutynin** is an **anticholinergic** agent that acts primarily as a competitive antagonist at **muscarinic acetylcholine receptors**, especially M3 receptors in the bladder. - By blocking these receptors, it relaxes the **detrusor muscle**, thereby reducing involuntary contractions and treating symptoms of **overactive bladder**. *Adrenergic receptor antagonist* - Adrenergic receptor antagonists (like **beta-blockers** or **alpha-blockers**) target epinephrine and norepinephrine receptors, which are involved in sympathetic nervous system responses. - These drugs are used for conditions like hypertension or benign prostatic hyperplasia, and their mechanism of action is distinct from oxybutynin's effect on muscarinic receptors. *Histaminic antagonist* - Histaminic antagonists (like **antihistamines**) block histamine receptors and are commonly used to treat allergic reactions or insomnia. - Oxybutynin does not primarily act on histamine receptors, although some anticholinergics can have mild antihistaminic properties. *Serotonergic antagonist* - Serotonergic antagonists block serotonin receptors and are used for conditions such as migraine, nausea, or psychiatric disorders. - Oxybutynin's therapeutic effects are not mediated through antagonism of serotonin receptors.

Question 584: Which of the following is an example of reversible carbamate?

A. Galantamine
B. Rivastigmine (Correct Answer)
C. Ambenonium
D. Propoxur

Explanation: ***Rivastigmine*** - **Rivastigmine** is a **carbamate derivative** used in the treatment of Alzheimer's disease and Parkinson's disease dementia [1]. - It acts as a **reversible acetylcholinesterase inhibitor** through carbamoylation of the enzyme active site. - While sometimes called "pseudo-irreversible" due to its **slower decarbamoylation rate** compared to other carbamates (giving it a duration of ~10 hours), it remains a **true reversible carbamate** as the enzyme eventually regenerates spontaneously. - This distinguishes it from organophosphates, which form nearly irreversible bonds requiring synthesis of new enzyme [2]. *Galantamine* - **Galantamine** is a **tertiary alkaloid** derived from plants, not a carbamate. - It functions as a **reversible cholinesterase inhibitor** and an **allosteric potentiator of nicotinic acetylcholine receptors**. - Its mechanism involves **competitive inhibition** at the enzyme active site, not carbamoylation. *Ambenonium* - **Ambenonium** is a **synthetic quaternary ammonium compound**, not a carbamate. - It is used as a **reversible anticholinesterase agent** in myasthenia gravis. - Its chemical structure and mechanism differ from carbamates. *Propoxur* - **Propoxur** is a **carbamate insecticide** with reversible anticholinesterase activity. - While technically a reversible carbamate, it is **not used therapeutically in humans** and is primarily of toxicological rather than pharmacological importance in medical practice.

Question 585: Which of the following is a depolarising neuromuscular blocker?

A. Succinylcholine (Correct Answer)
B. Pancuronium
C. Dexacurium
D. D-Tubocurarine

Explanation: ***Succinylcholine*** - **Succinylcholine** works by mimicking acetylcholine and binding to **nicotinic acetylcholine receptors** at the neuromuscular junction, causing initial depolarization and fasciculations, followed by sustained depolarization that prevents further muscle contraction. - Due to its unique mechanism, it is the only clinically used **depolarizing neuromuscular blocker** and is primarily used for rapid sequence intubation because of its fast onset and short duration of action. *Pancuronium* - **Pancuronium** is a **nondepolarizing neuromuscular blocker** that competitively antagonizes acetylcholine receptors at the neuromuscular junction, preventing depolarization without activating the receptors. - It has a slower onset and longer duration of action compared to depolarizing agents, making it unsuitable for rapid intubation. *Dexacurium* - **Dexacurium** is also a **nondepolarizing neuromuscular blocker**, structurally similar to atracurium. - It acts by competitively blocking acetylcholine receptors, leading to muscle relaxation without initial depolarization. *D-Tubocurarine* - **D-Tubocurarine** is an older, prototypical **nondepolarizing neuromuscular blocker** that works by competitively antagonizing acetylcholine receptors. - It is known for its potential to cause **histamine release**, leading to hypotension and bronchospasm, and is now rarely used clinically due to safer alternatives.

Question 586: What is the drug of choice for the management of hypertension in Phaeochromocytoma?

A. Phentolamine (reversible alpha blocker)
B. Esmolol (cardioselective beta1 blocker)
C. Labetalol (combined alpha and beta blocker)
D. Phenoxybenzamine (non-selective irreversible alpha blocker) (Correct Answer)

Explanation: ***Phenoxybenzamine*** - **Phenoxybenzamine** is a **non-selective, irreversible alpha-adrenergic blocker**, making it the drug of choice for managing hypertension in pheochromocytoma. Its **irreversible binding** prevents catecholamines from overstimulating alpha-receptors, ensuring sustained blood pressure control. - Pre-surgical alpha-blockade with phenoxybenzamine is crucial to prevent life-threatening **hypertensive crises** during tumor manipulation, reducing perioperative morbidity and mortality. *Phentolamine (reversible alpha blocker)* - While phentolamine is an **alpha-adrenergic blocker**, it is **reversible** and has a shorter duration of action compared to phenoxybenzamine. - It is typically used for managing **acute hypertensive crises** in pheochromocytoma, but not as the primary long-term preparation or maintenance therapy. *Esmolol (cardioselective beta1 blocker)* - Esmolol is a **beta-blocker** that should only be initiated **after adequate alpha-blockade** has been achieved. If used alone, it can lead to paradoxical hypertension by blocking vasodilator beta-2 receptors, leaving vasoconstrictive alpha-receptors unopposed. - It's mainly used to control **tachycardia** and **arrhythmias** once alpha-blockade is established, not as the initial treatment for hypertension. *Labetalol (combined alpha and beta blocker)* - Labetalol, although a **combined alpha and beta-blocker**, has a **greater beta-blocking effect** than alpha-blocking effect. - Like other beta-blockers, if administered before adequate alpha-blockade, it can precipitate a **hypertensive crisis** due to unopposed alpha-adrenergic stimulation.

Question 587: Which of the following is a nonselective beta adrenergic antagonist?

A. Atenolol
B. Bisoprolol
C. Esmolol
D. Nadolol (Correct Answer)

Explanation: ***Nadolol*** - **Nadolol** is well-known for its **non-selective beta-adrenergic blockade**, affecting both beta-1 and beta-2 receptors [1]. - This broad action makes it effective for conditions like **hypertension** and **angina** but can exacerbate conditions like asthma due to beta-2 blockade [1]. *Atenolol* - **Atenolol** is a **beta-1 selective antagonist**, primarily affecting the heart by decreasing heart rate and contractility. - It is often preferred in patients with **respiratory conditions** because it has less impact on bronchial smooth muscle (beta-2 receptors). *Bisoprolol* - **Bisoprolol** is a **highly selective beta-1 blocker** (cardioselective) at low to moderate doses, making it a common choice for heart failure and hypertension. - Its selectivity means it has a **lower risk of bronchospasm** compared to non-selective beta-blockers. *Esmolol* - **Esmolol** is an **ultrashort-acting, cardioselective (beta-1 selective) beta-blocker** administered intravenously [2]. - Its rapid onset and brief duration of action make it suitable for **acute management of supraventricular tachyarrhythmias** and perioperative hypertension [2].

Question 588: Which cholinergic drug acts on the heart by decreasing the levels of cAMP and opening K+ channels?

A. Methacholine (Correct Answer)
B. Oxotremorine
C. Bethanechol
D. DMPP

Explanation: ***Methacholine*** - Methacholine is a **muscarinic agonist** that acts on **M2 receptors** in the heart, leading to a decrease in **cAMP** levels and opening of **K+ channels**. - These actions result in **hyperpolarization** of cardiac cells and a reduction in heart rate and contractility. *Oxotremorine* - Oxotremorine is a **non-selective muscarinic agonist** known for its central nervous system effects, often used in research to induce *tremors* and study Parkinson's disease models. - While it activates muscarinic receptors, its primary clinical or pharmacological relevance is not in direct cardiac depression via **cAMP** and **K+ channel** modulation like methacholine. *Bethanechol* - Bethanechol is a **selective muscarinic agonist** primarily acting on the **M3 receptors** in the bladder and gastrointestinal tract, promoting smooth muscle contraction. - It has minimal cardiovascular effects at therapeutic doses and is therefore not the primary agent for cardiac rate and rhythm modulation via **cAMP** and **K+ channels**. *DMPP* - DMPP (**Dimethylphenylpiperazinium**) is a **nicotinic acetylcholine receptor agonist**, known for stimulating **ganglionic nicotinic receptors**. - Its effects are mediated through ganglionic stimulation and not directly by muscarinic receptor activation in the heart that decreases **cAMP** and opens **K+ channels**.

Question 589: A chronic smoker was on nicotine replacement therapy and clonidine tablets for smoking de-addiction. He stopped taking clonidine tablets and now presents with a headache. What is the reason behind this condition?

A. Postural hypotension
B. Receptor upregulation
C. Rebound hypertension (Correct Answer)
D. Receptor hypersensitivity

Explanation: ***Rebound hypertension*** - **Clonidine withdrawal** can cause a sudden surge in blood pressure due to increased sympathetic activity, leading to **rebound hypertension** and symptoms like headaches. - This occurs because chronic clonidine use suppresses sympathetic outflow, and its abrupt discontinuation unmasks this suppressed activity, causing a hypertensive crisis. *Postural hypotension* - **Postural hypotension** is a common side effect of clonidine due to its vasodilatory effects, causing blood pressure to drop when standing. - However, the patient's headache following clonidine cessation is more indicative of a **hypertensive event**, not hypotension. *Receptor upregulation* - **Receptor upregulation** refers to an increase in the number of receptors, often in response to prolonged antagonism or decreased ligand exposure. - While receptor changes occur, the primary mechanism of clonidine withdrawal is the **overcompensation** of the sympathetic nervous system, not simply an increased number of receptors. *Receptor hypersensitivity* - **Receptor hypersensitivity** implies an exaggerated response to a normal concentration of a neurotransmitter, which can contribute to withdrawal symptoms. - While it plays a role, the more immediate and critical cause of the headache is the rapid increase in blood pressure due to **rebound sympathetic activity**.

Question 590: Which of the following drugs is an alpha 2 agonist?

A. Apraclonidine (Correct Answer)
B. Timolol
C. PG analogues
D. Verapamil

Explanation: ***Apraclonidine*** - **Apraclonidine** is a synthetic **alpha-2 adrenergic agonist** that reduces aqueous humor production and increases uveoscleral outflow, thereby lowering intraocular pressure. - It is primarily used for the short-term treatment of **open-angle glaucoma** or ocular hypertension. *Timolol* - **Timolol** is a **non-selective beta-adrenergic blocker** that reduces aqueous humor production, leading to a decrease in intraocular pressure. - It does not act on alpha-2 receptors, distinguishing it from apraclonidine. *PG analogues* - **Prostaglandin analogues** (PG analogues) such as latanoprost, bimatoprost, and travoprost are primarily used to treat glaucoma by **increasing uveoscleral outflow** of aqueous humor. - They act on **prostaglandin F2α receptors**, not alpha-2 adrenergic receptors. *Verapamil* - **Verapamil** is a **calcium channel blocker** primarily used to treat hypertension, angina, and arrhythmias. - It acts by blocking calcium channels in vascular smooth muscle and the heart, and does not have significant alpha-2 adrenergic agonist activity.

Practice by Chapter

Cholinergic Agonists

Practice Questions

Cholinergic Antagonists

Practice Questions

Adrenergic Agonists

Practice Questions

Adrenergic Antagonists

Practice Questions

Ganglionic Agents

Practice Questions

Neuromuscular Blocking Agents

Practice Questions

Autonomic Drugs in Ophthalmology

Practice Questions

Autonomic Drugs in Cardiovascular Disease

Practice Questions

Autonomic Drugs in Respiratory Disease

Practice Questions

Autonomic Drugs in Urological Disorders

Practice Questions

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