Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 231: Epinephrine is used for all the following, except?

A. To prolong the action of local anaesthetics
B. As a topical haemostatic agent
C. Management of acute heart failure (Correct Answer)
D. Management of cardiac arrest

Explanation: **Explanation:** The correct answer is **C. Management of acute heart failure.** **1. Why Option C is correct (The Concept):** In acute heart failure (AHF), the primary goal is to improve cardiac output while reducing the workload on a failing heart. Epinephrine is a potent agonist at $\alpha_1$, $\beta_1$, and $\beta_2$ receptors. While its $\beta_1$ effect increases contractility, its $\alpha_1$ effect causes significant peripheral vasoconstriction, which **increases afterload**. This increased resistance makes it harder for the failing heart to pump blood, potentially worsening the failure and increasing myocardial oxygen demand, which can trigger arrhythmias or ischemia. For AHF, inotropes like **Dobutamine** (selective $\beta_1$) or **Milrinone** are preferred as they do not cause significant vasoconstriction. **2. Analysis of Incorrect Options:** * **A. Prolong action of local anaesthetics:** Epinephrine causes local vasoconstriction ($\alpha_1$), which reduces the systemic absorption of the anaesthetic. This increases the duration of action and reduces systemic toxicity. * **B. Topical haemostatic agent:** Due to its potent $\alpha_1$-mediated vasoconstriction, it is used topically to control bleeding from accessible mucous membranes (e.g., epistaxis) or during tooth extractions. * **D. Management of cardiac arrest:** Epinephrine is the **drug of choice** in ACLS protocols for cardiac arrest (VF/VT, Asystole, PEA). Its $\alpha_1$ effect increases diastolic blood pressure, improving coronary and cerebral perfusion pressure during CPR. **Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Epinephrine is the DOC for **Anaphylactic Shock** (1:1000 IM). * **Contraindication:** Never inject Epinephrine with local anaesthetics into "end-artery" areas (fingers, toes, nose, penis, ear lobes) due to the risk of gangrene. * **Metabolism:** It is rapidly metabolized by **COMT** and **MAO**; hence, it is not effective orally.

Question 232: First dose hypotension is commonly seen with which of the following medications?

A. Yohimbine
B. Prazosin (Correct Answer)
C. Atenolol
D. Methyldopa

Explanation: **Explanation:** **Prazosin** is a highly selective **Alpha-1 ($\alpha_1$) adrenergic blocker**. The "First Dose Effect" is a classic pharmacological phenomenon associated with this drug class. 1. **Why Prazosin is correct:** $\alpha_1$ receptors are primarily responsible for maintaining vascular tone through vasoconstriction. By blocking these receptors, Prazosin causes sudden peripheral vasodilation and a decrease in venous return. When a patient takes the first dose (or a significantly increased dose), the body cannot compensate quickly enough, leading to **severe orthostatic hypotension** and occasionally syncope (fainting). 2. **Why other options are incorrect:** * **Yohimbine:** This is a selective **$\alpha_2$ blocker**. It increases sympathetic outflow and can actually cause an *increase* in blood pressure and heart rate, rather than hypotension. * **Atenolol:** A selective **$\beta_1$ blocker**. While it lowers blood pressure, it does not typically cause the dramatic, sudden first-dose postural hypotension seen with alpha-blockers. * **Methyldopa:** A centrally acting **$\alpha_2$ agonist**. It lowers blood pressure gradually by reducing sympathetic outflow from the brain; it is not associated with the acute first-dose phenomenon. **High-Yield Clinical Pearls for NEET-PG:** * **Prevention:** To minimize the risk, the first dose of Prazosin should be **low (1 mg)** and administered **at bedtime** ("Bedtime dose"). * **Other Drugs:** Similar effects can be seen with other selective $\alpha_1$ blockers like **Terazosin** and **Doxazosin**. * **Clinical Use:** Prazosin is used for hypertension and Raynaud's phenomenon, while Tamsulosin (another $\alpha_1$ blocker) is more uroselective for Benign Prostatic Hyperplasia (BPH). * **Reflex Tachycardia:** Unlike non-selective alpha-blockers (e.g., Phenoxybenzamine), Prazosin causes less reflex tachycardia because it spares the $\alpha_2$ receptors (which regulate norepinephrine feedback).

Question 233: Which muscarinic receptor causes vasodilation?

A. M1
B. M2
C. M3 (Correct Answer)
D. M4

Explanation: **Explanation:** The correct answer is **M3**. **Mechanism of Vasodilation:** Muscarinic receptors are not typically found on the smooth muscle of blood vessels in a way that receives direct parasympathetic innervation. However, **M3 receptors** are present on the **vascular endothelium**. When these receptors are stimulated by a muscarinic agonist (like Acetylcholine), they activate the Gq-phospholipase C pathway, leading to an increase in intracellular calcium. This triggers the enzyme **Nitric Oxide Synthase (NOS)** to produce **Nitric Oxide (NO)**, also known as Endothelium-Derived Relaxing Factor (EDRF). NO diffuses into the adjacent vascular smooth muscle, increases cGMP, and causes relaxation (vasodilation). *Note: If the endothelium is damaged, M3 stimulation directly on smooth muscle causes vasoconstriction.* **Why other options are incorrect:** * **M1:** These are primarily "Neural" receptors located in the CNS and gastric parietal cells (increasing HCl secretion). They are not involved in systemic vasodilation. * **M2:** These are "Cardiac" receptors located in the SA and AV nodes. Their stimulation leads to a decrease in heart rate (negative chronotropy) and conduction velocity via Gi protein (decreasing cAMP). * **M4:** These are primarily located in the CNS and act via Gi proteins to inhibit adenylate cyclase; they do not play a role in peripheral vascular tone. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Receptors:** M1, M3, M5 are **Gq** coupled (Stimulatory); M2, M4 are **Gi** coupled (Inhibitory). * **The "Rule of 1, 2, 3":** M1 = Brain/Glands; M2 = Heart; M3 = Smooth Muscle/Exocrine Glands/Endothelium. * **Atropine** is a competitive antagonist for all these receptors; it can block the vasodilatory effect of injected choline esters.

Question 234: d-Tubocurarine acts by:

A. Inhibiting nicotinic receptors at the myoneural junction (Correct Answer)
B. Inhibiting nicotinic receptors at the autonomic ganglion
C. Producing a depolarizing block
D. Inhibiting the reuptake of acetylcholine

Explanation: ### Explanation **Mechanism of Action (The Correct Answer):** d-Tubocurarine (d-TC) is the prototype of **competitive (non-depolarizing) neuromuscular blockers**. It acts by binding to the **nicotinic acetylcholine receptors ($N_m$)** at the motor endplate of the myoneural junction. By competing with acetylcholine (ACh) for these sites, it prevents endplate depolarization, leading to flaccid muscle paralysis. This blockade can be reversed by increasing the concentration of ACh using acetylcholinesterase inhibitors like Neostigmine. **Analysis of Incorrect Options:** * **Option B:** While d-TC can block nicotinic receptors at autonomic ganglia ($N_n$ receptors) in high doses, its primary therapeutic and characteristic action is at the **neuromuscular junction**. Ganglionic blockade is considered a side effect, leading to hypotension. * **Option C:** Depolarizing blocks are produced by drugs like **Succinylcholine**. These agents act as agonists that cause persistent depolarization, making the membrane unresponsive to subsequent impulses. d-TC is a non-depolarizing blocker. * **Option D:** Inhibiting the reuptake of acetylcholine is not a standard mechanism for muscle relaxants. Hemicholinium is a research drug that inhibits choline uptake, but it is not d-TC's mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **Histamine Release:** d-TC is notorious for causing histamine release, which can lead to bronchospasm, skin flushing, and hypotension. * **Order of Paralysis:** Small, rapidly moving muscles (eyes, fingers) are affected first; the **diaphragm** is the last to be paralyzed and the first to recover. * **Antidote:** Neostigmine (usually co-administered with Atropine to prevent muscarinic side effects). * **Hoffman’s Elimination:** Remember that **Atracurium and Cisatracurium** are preferred in patients with renal/hepatic failure because they undergo spontaneous degradation (Hoffman's elimination), unlike d-TC.

Question 235: Which one of the following is FALSE regarding atropine?

A. Used to prevent adhesions in inflammatory conditions of the eye
B. Used in a concentration of 1%
C. Can be given in sinus bradycardia patients
D. Long acting miotic (Correct Answer)

Explanation: ### Explanation **Why Option D is the Correct Answer (The False Statement):** Atropine is a potent **competitive antagonist at muscarinic receptors**. In the eye, it blocks the $M_3$ receptors on the circular muscles (sphincter pupillae) of the iris. This leads to relaxation of the sphincter muscle, resulting in **Mydriasis** (dilation of the pupil), not miosis. Therefore, Atropine is a **long-acting Mydriatic** (and cycloplegic), with effects lasting up to 7–10 days. **Analysis of Incorrect Options (True Statements):** * **Option A:** In inflammatory conditions like iridocyclitis, atropine is used to prevent **synechiae** (adhesions between the iris and lens). By keeping the pupil dilated and the ciliary muscle paralyzed, it reduces pain and prevents the iris from sticking to the lens. * **Option B:** The standard clinical concentration of Atropine sulfate used in ophthalmic practice for adults is **1%** (drops or ointment). * **Option C:** Atropine is the **drug of choice for symptomatic sinus bradycardia**. It blocks the vagal (parasympathetic) input to the SA node, thereby increasing the heart rate. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cycloplegia:** Atropine causes paralysis of the ciliary muscle, leading to loss of accommodation. It is the most potent cycloplegic available. 2. **Contraindication:** Atropine is strictly contraindicated in patients with **Angle-Closure Glaucoma**, as mydriasis can further obstruct the aqueous outflow. 3. **Antidote:** Physostigmine (a tertiary amine) is the preferred antidote for atropine poisoning because it can cross the blood-brain barrier. 4. **Duration:** Atropine has the longest duration of action among mydriatics; for shorter procedures, Tropicamide (6 hours) or Cyclopentolate (24 hours) are preferred.

Question 236: Stimulation of which receptor will lead to the release of renin?

A. Alpha-1
B. Alpha-2
C. Beta-1 (Correct Answer)
D. Beta-2

Explanation: **Explanation:** The release of renin from the **Juxtaglomerular (JG) cells** of the kidney is primarily mediated by the sympathetic nervous system through **Beta-1 ($\beta_1$) receptors**. When these receptors are stimulated, they activate the Gs-protein-adenylyl cyclase pathway, increasing intracellular cAMP, which triggers the exocytosis of renin into the bloodstream. This is the rate-limiting step of the Renin-Angiotensin-Aldosterone System (RAAS). **Analysis of Options:** * **Beta-1 (Correct):** Predominantly located in the heart (increasing heart rate and contractility) and the JG cells of the kidney (stimulating renin release). * **Alpha-1 ($\alpha_1$):** Found mainly in vascular smooth muscle; stimulation causes vasoconstriction and pupillary dilation (mydriasis), but does not directly trigger renin release. * **Alpha-2 ($\alpha_2$):** These are primarily presynaptic inhibitory receptors. Stimulation in the CNS decreases sympathetic outflow, which would actually lead to a *decrease* in renin release. * **Beta-2 ($\beta_2$):** Primarily located in the bronchioles (bronchodilation) and skeletal muscle blood vessels (vasodilation). While they have metabolic effects, they are not the primary mediators of renin secretion. **High-Yield Clinical Pearls for NEET-PG:** 1. **Beta-Blockers and RAAS:** Non-selective and $\beta_1$-selective blockers (e.g., Propranolol, Atenolol) reduce blood pressure partly by inhibiting $\beta_1$-mediated renin release. 2. **Other Stimuli for Renin:** Apart from $\beta_1$ stimulation, renin is released in response to **decreased renal perfusion pressure** (baroreceptors in afferent arterioles) and **decreased sodium delivery** to the macula densa. 3. **Rule of 1:** Remember $\beta_1$ for **1** Heart and **1** Kidney (JG cells).

Question 237: What is the effect of sympathetic stimulation?

A. Pupillary constriction
B. Contraction of bladder detrusor
C. Bronchial smooth muscle contraction
D. Vasoconstriction of skin and mucus membranes (Correct Answer)

Explanation: The sympathetic nervous system (SNS) is the "fight or flight" system, designed to redirect blood flow to vital organs and prepare the body for physical exertion. ### **Explanation of the Correct Answer** **D. Vasoconstriction of skin and mucus membranes:** This occurs via the stimulation of **$\alpha_1$-adrenergic receptors**. During sympathetic activation, the body shunts blood away from non-essential peripheral areas (like the skin and mucosa) toward the skeletal muscles, heart, and brain. This is why a person may appear pale during a "fight or flight" response. ### **Analysis of Incorrect Options** * **A. Pupillary constriction:** This is a **parasympathetic** effect (miosis) mediated by $M_3$ receptors. Sympathetic stimulation causes **pupillary dilation (mydriasis)** via $\alpha_1$ receptors on the radial dilator pupillae muscle. * **B. Contraction of bladder detrusor:** This is a **parasympathetic** effect ($M_3$) to facilitate voiding. Sympathetic stimulation causes **relaxation** of the detrusor ($\beta_2/\beta_3$) and **contraction** of the internal sphincter ($\alpha_1$) to promote urinary retention. * **C. Bronchial smooth muscle contraction:** This is a **parasympathetic** effect ($M_3$). Sympathetic stimulation causes **bronchodilation** via **$\beta_2$ receptors** to increase airflow for oxygenation. ### **High-Yield NEET-PG Pearls** * **Dual Supply Exception:** Most blood vessels only have sympathetic innervation. Vasoconstriction is $\alpha_1$ mediated, while vasodilation in skeletal muscle is $\beta_2$ mediated. * **Sweat Glands:** Anatomically sympathetic but **cholinergic** (mediated by Acetylcholine acting on $M$ receptors). * **Mnemonic for SNS:** "Dilation" (Pupils, Bronchioles) and "Constriction" (Sphincters, Peripheral Vessels).

Question 238: Reflex tachycardia is most likely to occur after the systemic administration of which of the following drugs?

A. albuterol (Correct Answer)
B. methoxamine
C. phenylephrine
D. propranolol

Explanation: **Explanation:** The correct answer is **albuterol**. This question tests your understanding of the baroreceptor reflex and the vascular effects of adrenergic agonists. **1. Why Albuterol is Correct:** Albuterol is a selective **$\beta_2$-adrenergic agonist**. While primarily used for bronchodilation, systemic administration causes stimulation of $\beta_2$ receptors in the peripheral vasculature, leading to **vasodilation** and a decrease in peripheral vascular resistance (PVR). This drop in blood pressure triggers the **baroreceptor reflex**, resulting in a compensatory increase in sympathetic outflow to the heart (reflex tachycardia). Additionally, at higher doses, albuterol may lose selectivity and directly stimulate $\beta_1$ receptors in the heart, further increasing the heart rate. **2. Why the Other Options are Incorrect:** * **Methoxamine & Phenylephrine:** Both are selective **$\alpha_1$-adrenergic agonists**. They cause potent vasoconstriction, which increases peripheral vascular resistance and blood pressure. This rise in pressure triggers the baroreceptor reflex to increase vagal (parasympathetic) tone, leading to **reflex bradycardia**, not tachycardia. * **Propranolol:** This is a non-selective **$\beta$-blocker**. It blocks $\beta_1$ receptors in the heart, directly causing a decrease in heart rate (**bradycardia**). It would actually inhibit a reflex tachycardic response. **Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Drugs that cause significant vasodilation (e.g., Hydralazine, Nitrates, $\alpha$-blockers, $\beta_2$-agonists) typically cause reflex tachycardia. * **Vasoconstrictors** that increase mean arterial pressure (e.g., Phenylephrine, Norepinephrine) typically cause reflex bradycardia. * **Isoproterenol** ($\beta_1 + \beta_2$) causes the most profound tachycardia because it combines direct cardiac stimulation ($\beta_1$) with reflex tachycardia due to vasodilation ($\beta_2$).

Question 239: Which of the following is an alpha-1 blocker?

A. Tamsulosin (Correct Answer)
B. Prazosin
C. Terazosin
D. Clonidine

Explanation: **Explanation:** The question asks for an alpha-1 blocker, and while options A, B, and C are all technically alpha-1 antagonists, **Tamsulosin** is the most specific answer in many clinical contexts due to its subtype selectivity. **1. Why Tamsulosin is the Correct Answer:** Tamsulosin is a **selective alpha-1A blocker**. Alpha-1A receptors are primarily located in the smooth muscle of the bladder neck and prostate. By blocking these, Tamsulosin causes muscle relaxation, improving urine flow in patients with **Benign Prostatic Hyperplasia (BPH)**. Unlike non-selective alpha-1 blockers, it has minimal effect on alpha-1B receptors (found in blood vessels), resulting in less orthostatic hypotension. **2. Analysis of Other Options:** * **Prazosin & Terazosin:** These are **non-selective alpha-1 blockers** (blocking 1A, 1B, and 1D subtypes). While they are used for BPH, they are also potent antihypertensives. In many MCQ formats, if a "uroselective" drug like Tamsulosin is present, it is highlighted for its specific clinical utility in BPH. * **Clonidine:** This is an **alpha-2 agonist** (central acting). It stimulates alpha-2 receptors in the medulla, decreasing sympathetic outflow, and is used as an antihypertensive. **3. NEET-PG High-Yield Pearls:** * **First-Dose Phenomenon:** Prazosin is notorious for causing marked orthostatic hypotension and syncope after the first dose. * **Uroselectivity:** Tamsulosin and Silodosin are the preferred "uroselective" agents for BPH because they do not require dose titration and have minimal cardiovascular side effects. * **IFIS:** Tamsulosin is associated with **Intraoperative Floppy Iris Syndrome (IFIS)** during cataract surgery; patients should inform their ophthalmologist if they are on this medication.

Question 240: Which of the following statements regarding the use of adrenaline in anaphylactic shock is true?

A. The usual dose is 0.5-1 mg by intramuscular route. (Correct Answer)
B. Cerebral hemorrhage never occurs as an adverse effect to epinephrine when used in the treatment of anaphylactic shock.
C. It is repeated after every 2-4 hours.
D. The same solution can be given for subcutaneous as well as intravenous routes.

Explanation: Adrenaline (Epinephrine) is the **drug of choice** for anaphylactic shock due to its rapid onset and multi-receptor action ($\alpha_1, \alpha_2, \beta_1, \beta_2$). ### **Explanation of Options** * **Option A (Correct):** The standard adult dose for anaphylaxis is **0.5 mg (range 0.3–0.5 mg)** of a **1:1000** solution administered via the **Intramuscular (IM)** route in the anterolateral thigh. IM is preferred over subcutaneous (SC) because it achieves higher and faster peak plasma concentrations. * **Option B (Incorrect):** While rare if dosed correctly, **cerebral hemorrhage** and cardiac arrhythmias are known life-threatening adverse effects of adrenaline, usually occurring due to rapid spikes in blood pressure if administered too quickly or in excessive doses. * **Option C (Incorrect):** Adrenaline has a very short half-life. In anaphylaxis, if the patient does not improve, the dose should be repeated every **5–15 minutes**, not every 2–4 hours. * **Option D (Incorrect):** The concentrations differ significantly. The **1:1000 (1 mg/ml)** solution is used for IM/SC routes. However, for **Intravenous (IV)** use, it must be diluted to **1:10,000 (0.1 mg/ml)** or even 1:100,000 to prevent fatal arrhythmias and severe hypertension. ### **High-Yield NEET-PG Pearls** * **Mechanism in Anaphylaxis:** $\alpha_1$ (vasoconstriction to treat hypotension/edema), $\beta_1$ (positive inotropy), and $\beta_2$ (bronchodilation and stabilization of mast cells). * **Route of Choice:** IM (Anterolateral thigh/Vastus lateralis) is the gold standard. * **Concentration Ratios:** * **1:1,000** = 1 mg in 1 ml (IM use) * **1:10,000** = 1 mg in 10 ml (IV use in Cardiac Arrest) * **Pediatric Dose:** 0.01 mg/kg (max 0.3 mg).

Practice by Chapter

Cholinergic Agonists

Practice Questions

Cholinergic Antagonists

Practice Questions

Adrenergic Agonists

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Adrenergic Antagonists

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Ganglionic Agents

Practice Questions

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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