Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 291: Which of the following is a spasmolytic analgesic?

A. Dicyclomine (Correct Answer)
B. Physostigmine
C. Tropicamide
D. None of the above

Explanation: ### Explanation **1. Why Dicyclomine is the Correct Answer:** Dicyclomine is a **synthetic quaternary ammonium anticholinergic** drug. It acts as a direct smooth muscle relaxant and a competitive antagonist at muscarinic (M3) receptors. In the gastrointestinal tract, it reduces spasms of the smooth muscles, which effectively relieves the pain associated with intestinal colic or irritable bowel syndrome (IBS). Because it treats the cause of the pain (spasm) while providing relief, it is categorized as a **spasmolytic analgesic**. **2. Why the Other Options are Incorrect:** * **Physostigmine (Option B):** This is a tertiary amine **anticholinesterase** (parasympathomimetic). It increases acetylcholine levels at the synapse. Instead of relieving spasms, it would actually increase GI motility and potentially cause cramping. It is primarily used as an antidote for atropine poisoning. * **Tropicamide (Option C):** This is a short-acting **antimuscarinic** used exclusively in ophthalmology. It is applied topically to induce mydriasis (dilation of the pupil) and cycloplegia for fundus examination. It has no role as a systemic spasmolytic. **3. NEET-PG High-Yield Clinical Pearls:** * **Mechanism:** Dicyclomine has dual action—anticholinergic activity + direct non-specific smooth muscle relaxant activity. * **Clinical Use:** It is the drug of choice for **smooth muscle biliary, renal, or intestinal colic** and is frequently used in dysmenorrhea. * **Contraindications:** Like all anticholinergics, avoid in patients with **Glaucoma** (increases intraocular pressure) and **Prostatic Hypertrophy** (causes urinary retention). * **Other Spasmolytics:** Other drugs in this category include **Hyoscine (Scopolamine)** and **Drotaverine** (a PDE-4 inhibitor).

Question 292: Which of the following effects of adrenaline is reversed by prazosin but not by propranolol?

A. Tachycardia
B. Bronchial dilatation
C. Urinary symptoms of benign prostatic hyperplasia (BHP) (Correct Answer)
D. Migraine

Explanation: **Explanation:** The core concept tested here is the **receptor selectivity** of adrenergic agonists and antagonists. Adrenaline (Epinephrine) is a non-selective agonist acting on $\alpha_1, \alpha_2, \beta_1,$ and $\beta_2$ receptors. To identify an effect reversed by **Prazosin** but not **Propranolol**, we must look for a physiological response mediated specifically by **$\alpha_1$-receptors**. 1. **Why Option C is Correct:** In the bladder neck and prostate, $\alpha_1$-receptors (specifically $\alpha_{1A}$) mediate smooth muscle contraction, which worsens urinary obstruction in BPH. Adrenaline would cause contraction here. **Prazosin** is a selective $\alpha_1$-blocker; it relaxes these muscles, relieving symptoms. **Propranolol** is a non-selective $\beta$-blocker ($\beta_1 + \beta_2$) and has no effect on $\alpha_1$-mediated prostatic contraction. 2. **Analysis of Incorrect Options:** * **A. Tachycardia:** This is primarily mediated by **$\beta_1$-receptors** in the heart. It is reversed/blocked by Propranolol, not Prazosin. * **B. Bronchial dilatation:** This is mediated by **$\beta_2$-receptors**. Propranolol blocks this effect (potentially causing bronchospasm), while Prazosin has no significant effect on the airways. * **D. Migraine:** While Propranolol is used for migraine *prophylaxis* (mechanism involving $\beta$-blockade and vascular stability), Prazosin is not a standard treatment for migraine and does not reverse adrenaline's acute vascular effects in a way that treats the condition. **High-Yield Clinical Pearls for NEET-PG:** * **Dale’s Vasomotor Reversal:** This phenomenon occurs when adrenaline is given after an $\alpha$-blocker (like Prazosin). The $\alpha$-mediated vasoconstriction is blocked, leaving the $\beta_2$-mediated vasodilation unopposed, leading to a fall in blood pressure instead of a rise. * **Prazosin Side Effect:** Watch for the **"First Dose Phenomenon"** (marked postural hypotension). * **BPH Specificity:** Tamsulosin is preferred over Prazosin for BPH because it is more selective for the $\alpha_{1A}$ subtype found in the prostate, causing less systemic hypotension.

Question 293: Which drug can improve urinary flow rate in benign prostatic hypertrophy without affecting prostate size?

A. Finasteride
B. Prazosin (Correct Answer)
C. Goserelin
D. Amphetamine

Explanation: ### Explanation **Correct Option: B. Prazosin** Benign Prostatic Hypertrophy (BPH) causes urinary obstruction through two components: **Static** (increased prostate size) and **Dynamic** (increased smooth muscle tone in the bladder neck and prostatic urethra). Prazosin is a selective **$\alpha_1$-adrenergic blocker**. It works by relaxing the smooth muscles of the bladder neck and prostatic capsule (dynamic component). This reduces resistance to urine flow and improves symptoms rapidly. Crucially, $\alpha$-blockers **do not affect the size of the prostate gland** or serum PSA levels; they only address the functional obstruction. **Analysis of Incorrect Options:** * **A. Finasteride:** This is a **5-$\alpha$ reductase inhibitor**. It prevents the conversion of testosterone to dihydrotestosterone (DHT). Unlike Prazosin, it acts on the **static component** by actually **reducing the size of the prostate** over 6–12 months. * **C. Goserelin:** A **GnRH agonist** used primarily in prostate cancer. It causes medical castration by suppressing testosterone. While it can reduce prostate size, it is not a standard treatment for BPH due to its severe side effect profile (e.g., impotence, hot flashes). * **D. Amphetamine:** An indirect-acting sympathomimetic that increases norepinephrine release. This would stimulate $\alpha_1$ receptors, causing contraction of the bladder neck and potentially worsening urinary retention in a BPH patient. **High-Yield Clinical Pearls for NEET-PG:** * **First-line for BPH:** $\alpha_1$ blockers (e.g., Tamsulosin, Alfuzosin) are preferred because they provide **immediate** symptomatic relief. * **Uroselectivity:** **Tamsulosin** is an $\alpha_{1A}$ subtype selective blocker; it has less effect on systemic blood pressure compared to Prazosin (which can cause "first-dose hypotension"). * **Combination Therapy:** Finasteride + Doxazosin is often used to both improve flow (dynamic) and reduce size (static) to prevent disease progression. * **Side Effect:** Watch for **"Floppy Iris Syndrome"** during cataract surgery in patients taking Tamsulosin.

Question 294: Which of the following statements about Esmolol is true?

A. It is an alpha blocker.
B. It has a long half-life.
C. It is not cardioselective.
D. It is used in left ventricular decompensation. (Correct Answer)

Explanation: **Explanation:** **Esmolol** is a unique, ultra-short-acting **beta-1 selective (cardioselective) blocker**. **Why Option D is Correct:** While beta-blockers are generally contraindicated in acute heart failure, Esmolol is specifically indicated for the management of **supraventricular tachyarrhythmias** or hypertension in patients with **left ventricular decompensation** (acute heart failure) or perioperative settings. Its "safety net" lies in its ultra-short duration of action; if the patient’s cardiac output drops further, the drug effect disappears within minutes of stopping the infusion, allowing for precise titration. **Analysis of Incorrect Options:** * **Option A:** Esmolol is a **Beta-blocker**, not an alpha-blocker. * **Option B:** It has an **extremely short half-life** (approximately 9 minutes). This is because it is rapidly hydrolyzed by **red blood cell esterases** (not by liver or kidney enzymes). * **Option C:** It is highly **cardioselective** (Beta-1 selective), making it safer for patients with reactive airway disease compared to non-selective blockers like Propranolol. **High-Yield Clinical Pearls for NEET-PG:** * **Route of Administration:** Always given **Intravenously (IV)**; never orally. * **Metabolism:** Hydrolysis by RBC esterases (Unique feature). * **Indications:** Aortic dissection (to reduce shear stress), Thyroid storm, and perioperative tachycardia/hypertension. * **Mnemonic:** Remember **"Esmolol is Easy-off"** due to its rapid offset of action.

Question 295: All of the following actions of histamine are mediated through H1 receptors EXCEPT?

A. Release of EDRF from vascular endothelium resulting in vasodilation
B. Direct action on vascular smooth muscle causing vasodilation (Correct Answer)
C. Bronchoconstriction
D. Release of catecholamines from adrenal medulla

Explanation: Histamine exerts its physiological effects through four distinct G-protein coupled receptors (H1–H4). Understanding the dual mechanism of histamine-induced vasodilation is crucial for NEET-PG. **1. Why Option B is the Correct Answer:** Vasodilation caused by histamine involves two distinct pathways: * **Indirect (H1-mediated):** Histamine acts on H1 receptors on **vascular endothelial cells**, triggering the release of **Endothelium-Derived Relaxing Factor (EDRF/Nitric Oxide)**. This is rapid and short-lived. * **Direct (H2-mediated):** Histamine acts directly on **vascular smooth muscle** via **H2 receptors** (using the cAMP pathway). This effect is slower in onset but more sustained. Therefore, direct action on smooth muscle is an H2 effect, not H1. **2. Analysis of Incorrect Options:** * **Option A:** As noted above, H1 receptors on the endothelium are responsible for EDRF release. * **Option C:** H1 receptors are located on bronchial smooth muscle. Their activation leads to Gq-mediated phospholipase C activation, causing potent **bronchoconstriction** (clinically significant in asthma). * **Option D:** In the adrenal medulla, histamine triggers the release of catecholamines (Adrenaline/Noradrenaline) specifically through **H1 receptors**. **High-Yield Clinical Pearls for NEET-PG:** * **Triple Response of Lewis:** Consists of Red spot (H1/H2), Flare (H1), and Wheal (H1). * **Gastric Acid:** Mediated purely by **H2 receptors** on parietal cells. * **Triple Vaccine (DPT):** Can act as a histamine liberator. * **H3 Receptors:** Primarily presynaptic autoreceptors in the CNS that inhibit neurotransmitter release (e.g., Pitolisant used for narcolepsy).

Question 296: Which of the following statements is not true regarding Dobutamine?

A. Agonist of D1 and D2 receptors (Correct Answer)
B. Derivative of dopamine
C. Selective beta-agonistic action
D. Reduced chances of arrhythmia than adrenaline

Explanation: **Explanation** **Dobutamine** is a synthetic catecholamine primarily used as an inotropic agent in the management of acute heart failure and cardiogenic shock. **1. Why Option A is the Correct Answer (The False Statement):** Unlike its parent compound Dopamine, **Dobutamine does not act on Dopaminergic (D1 and D2) receptors.** It lacks the specific structural components required to stimulate renal or mesenteric vasodilation via D1 receptors. Therefore, it does not directly increase renal blood flow through dopaminergic pathways. **2. Analysis of Other Options:** * **Option B (Derivative of Dopamine):** This is true. Dobutamine is a structural analogue of dopamine, synthesized by substituting a large chemical group on the amino nitrogen. * **Option C (Selective Beta-agonistic action):** This is true. It acts predominantly on **$\beta_1$ receptors** in the heart to increase contractility (inotropy) with a relatively lesser effect on heart rate (chronotropy). It has minor $\beta_2$ and $\alpha_1$ activity, but the net effect is potent inotropy. * **Option D (Reduced chances of arrhythmia):** This is true. Compared to Adrenaline or Isoprenaline, Dobutamine is less likely to cause significant tachycardia or provoke life-threatening arrhythmias at therapeutic doses, making it safer for cardiac patients. **Clinical Pearls for NEET-PG:** * **Drug of Choice:** Dobutamine is the preferred inotrope for **Cardiogenic Shock** (where BP is relatively stable) and **Stress Echocardiography** (to identify hibernating myocardium). * **Isomerism:** It is a racemic mixture; the (+) isomer is a $\beta$-agonist, while the (-) isomer is an $\alpha_1$-agonist. * **Half-life:** It has a very short half-life (~2 minutes), requiring continuous IV infusion. * **Tolerance:** Tachyphylaxis (diminished response) can occur with prolonged use (more than 72 hours) due to receptor down-regulation.

Question 297: Action of which of the following drugs is rapidly terminated?

A. Succinylcholine (Correct Answer)
B. Atracurium
C. Pancuronium
D. D-tubocurarine

Explanation: **Explanation:** The correct answer is **Succinylcholine**. **Why Succinylcholine is correct:** Succinylcholine is a depolarizing neuromuscular blocker known for its **ultra-short duration of action** (typically 5–10 minutes). Its action is rapidly terminated because it is hydrolyzed by **Pseudocholinesterase (Butyrylcholinesterase)** found in the plasma. Unlike other neuromuscular blockers, it does not rely on hepatic metabolism or renal excretion for the termination of its primary effect, making it the drug of choice for rapid sequence intubation. **Why the other options are incorrect:** * **Atracurium:** This is an intermediate-acting non-depolarizing blocker (20–35 minutes). While it undergoes a unique "Hofmann elimination" (spontaneous non-enzymatic degradation), it is not as rapid as Succinylcholine. * **Pancuronium:** This is a long-acting steroid-based non-depolarizing blocker (duration >60 minutes). It is primarily eliminated by the kidneys and has a slow onset and offset. * **D-tubocurarine:** This is the prototype long-acting non-depolarizing blocker. It has a slow onset and a prolonged duration of action, largely due to its slow redistribution and renal excretion. **High-Yield Clinical Pearls for NEET-PG:** * **Succinylcholine Apnea:** Occurs in patients with a genetic deficiency of pseudocholinesterase, leading to prolonged paralysis. * **Phase II Block:** Occurs with high doses or continuous infusion of Succinylcholine, where the block changes from depolarizing to resembling a non-depolarizing block. * **Side Effects:** Watch for hyperkalemia (contraindicated in burn/trauma patients), malignant hyperthermia (treated with Dantrolene), and muscle fasciculations. * **Mnemonic:** Succinylcholine is "Succ-inct" (brief/short-acting).

Question 298: Which of the following is a phase I blocker?

A. Suxamethonium (Correct Answer)
B. Atracurium
C. Gallamine
D. D-Tubocurare

Explanation: ### Explanation Neuromuscular blocking agents (NMBAs) are classified into two main categories based on their mechanism of action at the nicotinic acetylcholine receptor (Nm) of the motor endplate: **Depolarizing** and **Non-depolarizing** blockers [1]. **1. Why Suxamethonium is Correct:** **Suxamethonium (Succinylcholine)** is the only clinically used **Depolarizing Neuromuscular Blocker** [2]. It acts as a **Phase I blocker** by mimicking acetylcholine; it binds to Nm receptors and causes persistent depolarization [2]. Because it is not metabolized by acetylcholinesterase (it is degraded by plasma pseudocholinesterase), the membrane remains depolarized and unresponsive to subsequent impulses, leading to flaccid paralysis preceded by fasciculations [3]. **2. Why the Other Options are Incorrect:** * **Atracurium, Gallamine, and D-Tubocurare:** These are all **Non-depolarizing Blockers** (Competitive Antagonists) [1]. They work by competing with acetylcholine for the Nm receptor without activating it. They do not cause initial depolarization or fasciculations and are often referred to as "pachycurares." **3. High-Yield Clinical Pearls for NEET-PG:** * **Phase II Block:** With prolonged or continuous infusion of Suxamethonium, the membrane repolarizes but becomes desensitized to acetylcholine [4]. This is known as a Phase II block, which clinically resembles a non-depolarizing block [4]. * **Metabolism:** Suxamethonium is metabolized by **Pseudocholinesterase** (Butyrylcholinesterase). Patients with an inherited deficiency of this enzyme experience **Succinylcholine Apnea**. * **Side Effects:** Important adverse effects include hyperkalemia (avoid in burn/trauma patients), muscle soreness, and it is a known trigger for **Malignant Hyperthermia** (Treated with Dantrolene). * **Atracurium:** Notable for undergoing **Hofmann Elimination** (spontaneous non-enzymatic degradation), making it safe in renal and hepatic failure.

Question 299: An example of covalent drug-receptor interaction is:

A. Noradrenaline binding to Beta 1 adrenergic receptor
B. Acetylcholine binding to muscarinic receptor
C. Prazosin binding to alpha 1 adrenergic receptor
D. Phenoxybenzamine binding to alpha adrenergic receptor (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Phenoxybenzamine binding to alpha adrenergic receptor.** **1. Why it is correct:** Most drug-receptor interactions are reversible and involve weak bonds (ionic, hydrogen, or Van der Waals). However, **Phenoxybenzamine** is a classic example of an **irreversible, non-competitive antagonist**. It undergoes a chemical transformation to form a highly reactive ethyleniminium intermediate, which then forms a **strong covalent bond** with the alpha-adrenergic receptor. Because covalent bonds are stable and difficult to break, the blockade is long-lasting (24–48 hours) and cannot be overcome by increasing the concentration of the agonist. **2. Why the other options are incorrect:** * **A & B (Noradrenaline and Acetylcholine):** These are endogenous neurotransmitters. Physiological signaling requires rapid onset and termination; therefore, they bind via **reversible, non-covalent bonds** to allow for quick dissociation and signal termination. * **C (Prazosin):** Unlike phenoxybenzamine, Prazosin is a **selective, competitive alpha-1 blocker**. It binds reversibly to the receptor, meaning its inhibitory effect can be overcome by increasing the concentration of an agonist (like Noradrenaline). **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Use:** Phenoxybenzamine is primarily used in the preoperative management of **Pheochromocytoma** to prevent hypertensive crises. * **The "Epinephrine Reversal" (Dale’s Vasomotor Reversal):** If Epinephrine is given after Phenoxybenzamine, the alpha-mediated vasoconstriction is blocked, leaving the beta-2 mediated vasodilation unopposed, resulting in a **fall** in blood pressure. * **Other Covalent Inhibitors:** Other high-yield examples of covalent/irreversible binding include **Aspirin** (COX enzymes), **Omeprazole** (H+/K+ ATPase), and **Organophosphates** (Acetylcholinesterase).

Question 300: All of the following are adverse effects of atropine, except:

A. Bronchoconstriction (Correct Answer)
B. Tachycardia
C. Dry mouth
D. Mydriasis

Explanation: **Explanation:** Atropine is a classic **competitive muscarinic antagonist**. To understand its adverse effects, one must remember that it blocks the "Rest and Digest" (parasympathetic) system, leading to "Sympathetic-like" effects. **Why Bronchoconstriction is the Correct Answer:** In the lungs, parasympathetic stimulation via **M3 receptors** causes bronchoconstriction and increased secretions. By blocking these receptors, Atropine actually causes **bronchodilation** and decreased secretions. Therefore, bronchoconstriction is not an adverse effect; rather, its opposite (bronchodilation) is a therapeutic effect used in conditions like COPD or during anesthesia. **Analysis of Incorrect Options:** * **Tachycardia:** Atropine blocks **M2 receptors** at the SA node, removing the vagal "brake" on the heart, which leads to an increased heart rate. * **Dry Mouth (Xerostomia):** Salivary glands are highly sensitive to muscarinic blockade. Atropine inhibits **M3-mediated** salivary secretion, making dry mouth one of the most common side effects. * **Mydriasis:** By blocking **M3 receptors** on the pupillary sphincter muscle, Atropine causes passive dilation of the pupil (mydriasis) and paralysis of accommodation (cycloplegia). **NEET-PG High-Yield Pearls:** * **Mnemonic for Atropine Toxicity:** "Hot as a hare (fever), Red as a beet (flushing), Dry as a bone (no sweat/saliva), Blind as a bat (cycloplegia), Mad as a hatter (delirium)." * **Drug of Choice:** Atropine is the DOC for **symptomatic bradycardia** and **organophosphate poisoning**. * **Contraindication:** It is strictly contraindicated in patients with **narrow-angle glaucoma** (due to mydriasis) and **Benign Prostatic Hyperplasia (BPH)** (due to urinary retention).

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free