Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 331: All of the following drugs act on dopaminergic receptors except?

A. Domperidone
B. Dobutamine (Correct Answer)
C. Fenoldopam
D. Metoclopramide

Explanation: **Explanation:** The correct answer is **Dobutamine** because its primary mechanism of action involves adrenergic receptors, not dopaminergic receptors. **1. Why Dobutamine is the correct answer:** Dobutamine is a synthetic catecholamine that acts as a **selective $\beta_1$-agonist**. While it has minor effects on $\beta_2$ and $\alpha_1$ receptors, it has **no significant activity at dopamine (D) receptors**. It is primarily used as an inotropic agent in cardiogenic shock and acute heart failure to increase cardiac output without significantly increasing heart rate. **2. Analysis of other options:** * **Domperidone:** This is a peripheral **$D_2$ receptor antagonist**. It is used as an antiemetic and prokinetic. Unlike metoclopramide, it does not cross the blood-brain barrier easily, resulting in fewer extrapyramidal side effects. * **Fenoldopam:** This is a selective **$D_1$ receptor agonist**. It causes systemic vasodilation, particularly in the renal and mesenteric beds. It is used clinically in the management of hypertensive emergencies. * **Metoclopramide:** This is a central and peripheral **$D_2$ receptor antagonist**. It also possesses $5-HT_4$ agonist properties. It is a potent antiemetic and prokinetic agent but can cause extrapyramidal symptoms (EPS) due to central dopamine blockade. **Clinical Pearls for NEET-PG:** * **Dopamine (The Drug):** Unlike Dobutamine, Dopamine acts on $D_1$ receptors (low dose), $\beta_1$ receptors (medium dose), and $\alpha_1$ receptors (high dose). * **Renal Dose Myth:** Low-dose dopamine was historically thought to protect the kidneys via $D_1$ vasodilation, but clinical trials (SOAP II) have shown no mortality benefit in acute renal failure. * **Drug of Choice:** Fenoldopam is unique as it is the only parenteral dopamine agonist used for hypertensive crisis, especially when renal perfusion is a concern.

Question 332: Which of the following drugs is not given alone in a patient of pheochromocytoma?

A. Atenolol (Correct Answer)
B. Prazosin
C. Nitroprusside
D. Metyrosine

Explanation: In **Pheochromocytoma**, there is a massive release of catecholamines (Epinephrine and Norepinephrine). These act on both **$\alpha$-receptors** (causing vasoconstriction and hypertension) and **$\beta$-receptors** (causing tachycardia and vasodilation in skeletal muscle). ### **Why Atenolol (Option A) is the Correct Answer** Atenolol is a selective $\beta_1$-blocker. If a $\beta$-blocker is given alone, it inhibits $\beta_2$-mediated vasodilation while leaving $\alpha_1$-mediated vasoconstriction unopposed. This leads to a **paradoxical rise in blood pressure** (hypertensive crisis). * **Rule:** In pheochromocytoma, **always give an $\alpha$-blocker first**, followed by a $\beta$-blocker. ### **Why Other Options are Incorrect** * **B. Prazosin:** This is a selective $\alpha_1$-blocker. It is safe and often used to control hypertension in these patients by reducing peripheral vascular resistance. * **C. Nitroprusside:** This is a potent vasodilator used intravenously to manage **hypertensive emergencies** during surgery for pheochromocytoma. * **D. Metyrosine:** This drug inhibits **Tyrosine Hydroxylase** (the rate-limiting enzyme in catecholamine synthesis). It is used to decrease the total catecholamine pool before surgery. ### **Clinical Pearls for NEET-PG** 1. **Drug of Choice (Pre-op):** Phenoxybenzamine (a non-selective, irreversible $\alpha$-blocker). 2. **The "7-10 Day Rule":** Start $\alpha$-blockade at least 7–10 days before surgery; add $\beta$-blockers only after adequate $\alpha$-blockade is achieved. 3. **Diagnosis:** Best initial screening test is **Urinary/Plasma Metanephrines**. 4. **Rule of 10s:** 10% are bilateral, 10% are malignant, 10% are extra-adrenal (Paraganglioma), and 10% are familial.

Question 333: All of the following are endogenous catecholamines except?

A. Epinephrine
B. Norepinephrine
C. Dopamine
D. Dobutamine (Correct Answer)

Explanation: **Explanation:** The distinction between catecholamines lies in their origin: **Endogenous** (naturally produced in the body) versus **Synthetic** (man-made for pharmacological use). **Why Dobutamine is the correct answer:** Dobutamine is a **synthetic** catecholamine. It is a structural analogue of dopamine developed specifically to act as a relatively selective **$\beta_1$-agonist**. Unlike endogenous catecholamines, it is used clinically to increase cardiac output in acute heart failure and cardiogenic shock without significantly affecting blood pressure or heart rate compared to other vasopressors. **Analysis of Incorrect Options:** * **A, B, and C (Epinephrine, Norepinephrine, Dopamine):** These are the three primary **endogenous** catecholamines. They are synthesized in the body from the amino acid **Tyrosine**. * **Dopamine** is the precursor to Norepinephrine. * **Norepinephrine** is the primary neurotransmitter of postganglionic sympathetic nerves. * **Epinephrine** is the main hormone secreted by the adrenal medulla. **High-Yield Clinical Pearls for NEET-PG:** * **Chemical Structure:** Catecholamines consist of a catechol nucleus (benzene ring with two hydroxyl groups) and an amine side chain. * **Metabolism:** All catecholamines (endogenous and synthetic) are rapidly metabolized by **COMT** (Catechol-O-methyltransferase) and **MAO** (Monoamine oxidase), which is why they have a short half-life and cannot be given orally. * **Isoprenaline** is another common example of a **synthetic** catecholamine often tested in exams. * **Dobutamine Clinical Note:** It is the drug of choice for **Stress Echocardiography** to provoke ischemia in patients unable to exercise.

Question 334: Which one of the following drugs does not induce mydriasis?

A. Phentolamine (Correct Answer)
B. Ephedrine
C. Phenylephrine
D. Cocaine

Explanation: To understand mydriasis (pupillary dilation), one must remember that it is mediated by the **Alpha-1 ($\alpha_1$) receptors** located on the radial (dilator) muscle of the iris. ### **1. Why Phentolamine is the Correct Answer** **Phentolamine** is a non-selective **alpha-adrenergic blocker** (blocks both $\alpha_1$ and $\alpha_2$). By blocking $\alpha_1$ receptors on the iris dilator muscle, it prevents contraction, thereby inhibiting mydriasis. In fact, alpha-blockers can lead to miosis (pupillary constriction) due to unopposed parasympathetic action. ### **2. Why the Other Options are Incorrect** * **Phenylephrine (Option C):** A selective **$\alpha_1$ agonist**. It directly stimulates the radial muscle, causing "active mydriasis" without affecting accommodation (no cycloplegia). * **Ephedrine (Option B):** A **mixed-acting sympathomimetic**. It acts directly on $\alpha$ and $\beta$ receptors and indirectly by releasing stored norepinephrine, leading to pupillary dilation. * **Cocaine (Option D):** An **indirect sympathomimetic**. It blocks the reuptake of norepinephrine at the synaptic cleft. The increased concentration of norepinephrine in the neuro-effector junction of the iris results in mydriasis. ### **3. High-Yield Clinical Pearls for NEET-PG** * **Active vs. Passive Mydriasis:** Sympathomimetics cause *active* mydriasis (contraction of dilator). Anticholinergics (like Atropine) cause *passive* mydriasis (relaxation of sphincter) accompanied by **cycloplegia** (loss of accommodation). * **Diagnostic Use:** Phenylephrine is preferred for routine fundus examination when cycloplegia is not required. * **Cocaine Test:** Historically used to diagnose **Horner’s Syndrome**. A normal eye dilates with cocaine, but a Horner's eye (sympathetic denervation) does not. * **Phentolamine Use:** Clinically used in the management of pheochromocytoma and extravasation of catecholamines.

Question 335: Which of the following drugs increases gastrointestinal motility?

A. Glycopyrrolate
B. Atropine
C. Neostigmine (Correct Answer)
D. Fentanyl

Explanation: **Explanation:** The gastrointestinal (GI) tract is primarily regulated by the parasympathetic nervous system via **muscarinic (M3) receptors**. Activation of these receptors increases intestinal tone and peristalsis while relaxing sphincters, thereby promoting GI motility. **Why Neostigmine is Correct:** Neostigmine is an **indirect-acting cholinergic agonist** (Anticholinesterase). It inhibits the enzyme acetylcholinesterase, leading to an accumulation of endogenous acetylcholine at the neuromuscular junction and neuroeffector sites. By increasing acetylcholine levels at M3 receptors, it stimulates intestinal smooth muscle contraction. Clinically, it is used to treat paralytic ileus and Ogilvie’s syndrome (acute colonic pseudo-obstruction). **Why Other Options are Incorrect:** * **Glycopyrrolate & Atropine:** These are **muscarinic antagonists** (Anticholinergics). They block M3 receptors in the gut, leading to decreased secretions and reduced motility (antispasmodic effect). They are often used pre-operatively to reduce secretions or to treat bradycardia. * **Fentanyl:** This is a potent **opioid analgesic**. Opioids act on $\mu$-receptors in the myenteric plexus to inhibit acetylcholine release, significantly *decreasing* GI motility and causing constipation. **High-Yield Clinical Pearls for NEET-PG:** * **Neostigmine Side Effects:** Being a quaternary ammonium compound, it does not cross the Blood-Brain Barrier (BBB), but it can cause systemic "DUMBELS" effects (Diarrhea, Urination, Miosis, Bradycardia, etc.). * **Prokinetic of Choice:** While Neostigmine increases motility, **Metoclopramide** (D2 antagonist) is more commonly used as a prokinetic for GERD and gastroparesis. * **Atropine vs. Glycopyrrolate:** Glycopyrrolate is preferred in anesthesia because it is polar, does not cross the BBB, and causes less tachycardia than atropine.

Question 336: Which of the following is not an endogenous catecholamine?

A. Dopamine
B. Dobutamine (Correct Answer)
C. Adrenaline
D. Noradrenaline

Explanation: **Explanation:** The core concept here is the distinction between **endogenous catecholamines** (naturally produced in the body) and **synthetic catecholamines** (man-made pharmacological agents). **Why Dobutamine is the correct answer:** **Dobutamine** is a **synthetic** catecholamine. It is a derivative of dopamine but is not produced naturally within the human body. Clinically, it acts primarily as a selective **$\beta_1$-agonist** with mild $\beta_2$ and $\alpha_1$ activity, making it a "positive inotrope" used in the management of acute heart failure and cardiogenic shock. **Why the other options are incorrect:** * **Dopamine (A):** An endogenous catecholamine and a precursor to Noradrenaline. It acts on D1, D2, $\beta_1$, and $\alpha_1$ receptors depending on the dose. * **Adrenaline/Epinephrine (C):** The primary hormone secreted by the **adrenal medulla**. It acts on all $\alpha$ and $\beta$ receptors. * **Noradrenaline/Norepinephrine (D):** The primary neurotransmitter of the **postganglionic sympathetic nerves**. It acts mainly on $\alpha_1$, $\alpha_2$, and $\beta_1$ receptors (with negligible effect on $\beta_2$). **High-Yield NEET-PG Pearls:** 1. **Chemical Structure:** Catecholamines consist of a catechol nucleus (benzene ring with two adjacent hydroxyl groups) and an amine side chain. 2. **Biosynthesis Pathway:** Tyrosine $\rightarrow$ L-Dopa $\rightarrow$ **Dopamine** $\rightarrow$ **Noradrenaline** $\rightarrow$ **Adrenaline**. 3. **Metabolism:** Endogenous catecholamines are rapidly metabolized by **MAO** (Monoamine oxidase) and **COMT** (Catechol-O-methyltransferase), which is why they have a very short half-life and are ineffective orally. 4. **Other Synthetic Catecholamines:** Isoprenaline (Isoproterenol) is another common synthetic catecholamine frequently tested alongside Dobutamine.

Question 337: Which of the following is a beta-antagonist?

A. Salbutamol
B. Propranolol (Correct Answer)
C. Salmeterol
D. Albuterol

Explanation: **Explanation:** The correct answer is **Propranolol**. **1. Why Propranolol is correct:** Propranolol is a prototype **non-selective beta-adrenergic antagonist** (beta-blocker). It works by competitively blocking both $\beta_1$ and $\beta_2$ receptors. By inhibiting these receptors, it decreases heart rate and contractility ($\beta_1$ effect) and can cause bronchoconstriction ($\beta_2$ effect). It is lipid-soluble, allowing it to cross the blood-brain barrier, which explains its use in prophylaxis for migraines and performance anxiety. **2. Why the other options are incorrect:** * **Salbutamol (Option A) & Albuterol (Option D):** These are the same drug (Albuterol is the US name). They are **Short-Acting Beta-2 Agonists (SABA)**. They stimulate $\beta_2$ receptors in the bronchial smooth muscle to cause bronchodilation, making them first-line treatments for acute asthma attacks. * **Salmeterol (Option C):** This is a **Long-Acting Beta-2 Agonist (LABA)**. It is used for the maintenance treatment of asthma and COPD but is not suitable for acute relief due to its slow onset of action. **3. High-Yield Clinical Pearls for NEET-PG:** * **Classification:** Beta-blockers ending in **-olol** are generally antagonists. * **Cardioselectivity:** Remember the mnemonic **"A to M"** (e.g., Atenolol, Metoprolol) for $\beta_1$ selective blockers, which are safer in patients with asthma. * **ISA:** Pindolol and Acebutolol possess **Intrinsic Sympathomimetic Activity**, meaning they cause less bradycardia at rest. * **Vasodilatory Beta-blockers:** Carvedilol and Labetalol block both $\alpha$ and $\beta$ receptors, while Nebivolol increases Nitric Oxide (NO) release. * **Contraindication:** Non-selective beta-blockers like Propranolol are strictly contraindicated in patients with **Asthma or COPD** due to the risk of life-threatening bronchospasm.

Question 338: Which of the following is a parasympatholytic agent?

A. Atropine (Correct Answer)
B. Neostigmine
C. Pyridostigmine
D. Acetylcholine

Explanation: ### Explanation **Correct Answer: A. Atropine** **Mechanism and Concept:** A **parasympatholytic** agent (also known as an anticholinergic or muscarinic antagonist) is a drug that reduces the activity of the parasympathetic nervous system. **Atropine** is the prototype competitive antagonist at muscarinic receptors. It blocks the action of acetylcholine (ACh) at effector organs, leading to effects such as tachycardia, mydriasis, decreased secretions, and bronchodilation. **Analysis of Incorrect Options:** * **B. Neostigmine & C. Pyridostigmine:** These are **parasympathomimetics**. Specifically, they are reversible acetylcholinesterase (AChE) inhibitors. By preventing the breakdown of endogenous ACh, they increase cholinergic activity. They are used in treating Myasthenia Gravis and reversing neuromuscular blockade. * **D. Acetylcholine:** This is the primary neurotransmitter of the parasympathetic nervous system. It is a direct-acting **parasympathomimetic** that stimulates both muscarinic and nicotinic receptors. **High-Yield Clinical Pearls for NEET-PG:** * **Atropine Flush:** High doses of atropine can cause cutaneous vasodilation (redness), especially in children. * **Drug of Choice (DOC):** Atropine is the DOC for **symptomatic sinus bradycardia** and **organophosphate poisoning** (where it antagonizes muscarinic effects). * **Contraindication:** Parasympatholytics like atropine are strictly contraindicated in patients with **narrow-angle glaucoma** (as they can precipitate an acute attack) and **Benign Prostatic Hyperplasia (BPH)** (due to risk of urinary retention). * **Mnemonic for Atropine Toxicity:** "Hot as a hare, red as a beet, dry as a bone, blind as a bat, and mad as a hatter."

Question 339: Cholinomimetics are used in which of the following conditions?

A. Glaucoma
B. Myasthenia gravis
C. Postoperative atony
D. All of the above (Correct Answer)

Explanation: **Explanation:** Cholinomimetics (Parasympathomimetics) are drugs that mimic the action of acetylcholine (ACh) by either directly stimulating cholinergic receptors or indirectly inhibiting the enzyme acetylcholinesterase. **Why "All of the above" is correct:** 1. **Glaucoma (Option A):** Direct agonists like **Pilocarpine** and anticholinesterases like **Physostigmine** are used. They cause miosis (contraction of the iris sphincter) and contraction of the ciliary muscle. This opens the canal of Schlemm and improves the drainage of aqueous humor, thereby reducing intraocular pressure. 2. **Myasthenia Gravis (Option B):** This is an autoimmune disorder characterized by a deficiency of functional nicotinic receptors (Nm) at the neuromuscular junction. Reversible anticholinesterases like **Pyridostigmine** (drug of choice) and **Neostigmine** increase the concentration of ACh at the synapse, improving muscle strength. 3. **Postoperative Atony (Option C):** Postoperative urinary retention and paralytic ileus are treated with drugs like **Bethanechol** (a choline ester) or **Neostigmine**. These agents stimulate muscarinic (M3) receptors in the bladder and GI tract, promoting detrusor contraction and increased intestinal motility. **High-Yield Clinical Pearls for NEET-PG:** * **Edrophonium:** A very short-acting anticholinesterase used in the **Tensilon Test** to diagnose Myasthenia Gravis (though now largely replaced by antibody testing). * **Pilocarpine:** Drug of choice for the emergency treatment of **Acute Angle Closure Glaucoma**. * **Cevimeline:** A specific M3 agonist used for **Sjögren’s syndrome** to increase salivation. * **Contraindications:** Cholinomimetics should be avoided in patients with **Asthma** (causes bronchoconstriction) and **Peptic Ulcer Disease** (increases gastric acid secretion).

Question 340: All of the following effects are seen with cholinergic muscarinic receptor stimulation except?

A. Sweating
B. Rise in blood pressure (Correct Answer)
C. Bradycardia
D. Urination

Explanation: **Explanation:** The correct answer is **B. Rise in blood pressure**. Muscarinic receptor (M1-M5) stimulation primarily activates the parasympathetic nervous system. Stimulation of **M3 receptors** on vascular endothelial cells triggers the release of **Nitric Oxide (NO)**, which leads to vasodilation and a **fall in blood pressure**, not a rise. While high doses of acetylcholine can cause a rise in BP via nicotinic receptors in sympathetic ganglia (the "Nicotinic effect of ACh"), pure muscarinic stimulation results in hypotension. **Analysis of other options:** * **A. Sweating:** Although sweat glands are part of the sympathetic nervous system anatomically, they are unique because they are **cholinergic** (mediated by **M3 receptors**). Thus, muscarinic agonists increase sweating (diaphoresis). * **C. Bradycardia:** Stimulation of **M2 receptors** in the SA node of the heart leads to hyperpolarization and a decrease in heart rate (negative chronotropic effect). * **D. Urination:** Muscarinic stimulation (**M3 receptors**) causes contraction of the detrusor muscle and relaxation of the trigone/sphincter, facilitating voiding of urine. **NEET-PG High-Yield Pearls:** * **Mnemonic for Cholinergic effects:** **DUMBELS** (Diarrhea, Urination, Miosis, Bradycardia/Bronchospasm, Emesis, Lacrimation, Salivation/Sweating). * **M1:** Gastric glands (CNS); **M2:** Heart; **M3:** Smooth muscles, Exocrine glands, and Eye (Ciliary muscle/Sphincter pupillae). * **Vascular Paradox:** Blood vessels have M3 receptors but no parasympathetic innervation. They respond only to exogenous muscarinic agonists or circulating ACh.

Practice by Chapter

Cholinergic Agonists

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Cholinergic Antagonists

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Adrenergic Agonists

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Adrenergic Antagonists

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Ganglionic Agents

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Neuromuscular Blocking Agents

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Autonomic Drugs in Ophthalmology

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Autonomic Drugs in Cardiovascular Disease

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Autonomic Drugs in Respiratory Disease

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Autonomic Drugs in Urological Disorders

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