Autonomic Nervous System Drugs Practice Questions

Q: What is true about phenylephrine?

Increases peripheral resistance. **Explanation:** **Phenylephrine** is a potent, selective **$\alpha_1$-adrenergic agonist** with negligible effects on $\beta$-receptors [1]. **1. Why Option C is Correct:** Phenylephrine acts directly on $\alpha_1$ receptors located in the vascular smooth muscle [2]. Activation of these receptors leads to significant **vasoconstriction** of both arterioles and venules [1]. This results in a marked **increase in Total Peripheral Resistance (TPR)** and a subsequent rise in both systolic and diastolic blood pressure [1]. **2. Why Other Options are Incorrect:** * **Option A:** Phenylephrine causes **hypertension**, not hypotension, due to its vasoconstrictive properties. It is often used clinically to treat distributive shock (e.g., neurogenic shock). * **Option B:** While it doesn't directly depress the myocardium, the rapid rise in blood pressure triggers a **reflex bradycardia** (via the baroreceptor reflex). This decrease in heart rate is a compensatory mechanism, not direct cardiac depression. * **Option D:** Phenylephrine causes **mydriasis** (dilation of the pupil) by contracting the radial dilator pupillae muscle ($\alpha_1$) [2]. Unlike atropine, it does not cause cycloplegia [3]. Importantly, it **decreases intraocular pressure** in some cases by constricting ciliary body blood vessels, which reduces aqueous humor production. **Clinical Pearls for NEET-PG:** * **Drug of Choice:** Phenylephrine is the preferred agent for producing mydialsis when cycloplegia (paralysis of accommodation) is not desired [3]. * **Reflex Bradycardia:** This is a classic "catch" in exams; remember that $\alpha_1$ agonists increase BP but *decrease* HR via the vagal reflex. * **Nasal Decongestant:** Used topically for relief, but chronic use can lead to **rhinitis medicamentosa** (rebound congestion).

Q: Exocytic release of acetylcholine (ACH) is blocked by which of the following?

Botulinum toxin. **Explanation:** The correct answer is **Botulinum toxin**. The release of Acetylcholine (ACh) from the presynaptic nerve terminal occurs via exocytosis, a process mediated by **SNARE proteins** (e.g., synaptobrevin, syntaxin, and SNAP-25). Botulinum toxin acts as a protease that cleaves these SNARE proteins, preventing the fusion of synaptic vesicles with the presynaptic membrane, thereby blocking the exocytic release of ACh. This leads to flaccid paralysis. **Analysis of Incorrect Options:** * **A. Hemicholinium:** This drug blocks the **rate-limiting step** of ACh synthesis by inhibiting the high-affinity sodium-dependent **choline transporter (CHT)**, preventing the uptake of choline into the nerve terminal. * **C. Alpha-bungarotoxin:** This is a snake venom toxin that acts **postsynaptically**. It binds irreversibly to nicotinic acetylcholine receptors (Nm) at the neuromuscular junction, blocking neurotransmission. * **D. Vesamicol:** This agent inhibits the **VAT (Vesicle-Associated Transporter)**, preventing the storage of ACh into synaptic vesicles. **NEET-PG High-Yield Pearls:** * **Black Widow Spider Venom (Alpha-latrotoxin):** Causes massive, explosive release of ACh (opposite of Botulinum). * **Lambert-Eaton Syndrome:** Antibodies against P/Q-type voltage-gated calcium channels also inhibit ACh release. * **Clinical use of Botulinum (Botox):** Used for focal dystonias, achalasia cardia, hyperhidrosis, and cosmetic reduction of wrinkles. * **Mnemonic for Synthesis/Storage/Release:** **H**emicholinium (Uptake), **V**esamicol (Storage), **B**otulinum (Release).

Q: Which of the following is the earliest and shortest-acting skeletal muscle relaxant?

Suxamethonium. ### Explanation **Correct Answer: D. Suxamethonium (Succinylcholine)** **Mechanism and Rationale:** Suxamethonium is the only **depolarizing neuromuscular blocker** used clinically. It works by mimicking the action of acetylcholine at the nicotinic receptors of the motor endplate, causing persistent depolarization. * **Onset:** It has the fastest onset of action (**30–60 seconds**), making it the gold standard for Rapid Sequence Induction (RSI). * **Duration:** It is the shortest-acting relaxant because it is rapidly hydrolyzed by **pseudocholinesterase (plasma cholinesterase)** in the blood. Its clinical effect lasts only **5–10 minutes**. **Analysis of Incorrect Options:** All other options belong to the **Non-depolarizing** class (competitive antagonists), which generally have a slower onset and longer duration than Suxamethonium: * **A. Rocuronium:** An aminosteroid with the fastest onset among non-depolarizers (60–90 seconds), but its duration is intermediate (30–40 minutes). * **B. Vecuronium:** An intermediate-acting aminosteroid (duration 30–40 minutes) with a slower onset (2–3 minutes) than Rocuronium. * **C. Atracurium:** An intermediate-acting benzylisoquinolinium (duration 30–45 minutes). It is unique for its metabolism via **Hofmann elimination**, making it safe in liver and kidney failure. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** For Rapid Sequence Induction (RSI) and when a "difficult airway" is anticipated (due to short duration). * **Side Effects:** Muscle fasciculations (prevented by a "pre-curarization" dose), hyperkalemia (avoid in burns/trauma), and it is a potent trigger for **Malignant Hyperthermia**. * **Prolonged Apnea:** Occurs in patients with **pseudocholinesterase deficiency** (diagnosed by a low Dibucaine Number). * **Mivacurium:** The shortest-acting *non-depolarizing* muscle relaxant, but it is still slower and longer-acting than Suxamethonium.

Q: Which of the following is an ultrashort acting muscle relaxant?

Succinyl choline. **Explanation:** **Succinylcholine (Suxamethonium)** is the correct answer because it is the only **depolarizing neuromuscular blocker** in clinical use and is classified as **ultrashort-acting**. Its rapid onset (30–60 seconds) and brief duration of action (5–10 minutes) occur because it is rapidly hydrolyzed by **pseudocholinesterase (plasma cholinesterase)** in the blood. This makes it the gold standard for **Rapid Sequence Induction (RSI)** and short procedures like electroconvulsive therapy (ECT). **Analysis of Incorrect Options:** * **Rocuronium (Option A):** An aminosteroid non-depolarizing blocker. It has a rapid onset (similar to Succinylcholine at higher doses) but is classified as **intermediate-acting** (30–40 minutes). * **Atracurium (Option B):** A benzylisoquinolone non-depolarizing blocker. It is **intermediate-acting** and unique for its metabolism via **Hofmann elimination** (spontaneous degradation), making it safe in renal or hepatic failure. * **Doxacurium (Option C):** A potent non-depolarizing blocker that is **long-acting** (>60 minutes). It is rarely used today due to the preference for shorter-acting agents. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Duration:** *Shortest to Longest* → Succinylcholine < Mivacurium < Atracurium/Vecuronium/Rocuronium < Pancuronium/Doxacurium. * **Side Effects of Suxamethonium:** Hyperkalemia (avoid in burns/trauma), muscle fasciculations, myalgia, and it is a known trigger for **Malignant Hyperthermia**. * **Phase II Block:** Occurs with prolonged or repeated doses of Succinylcholine, where the block begins to resemble a non-depolarizing block. * **Dibucaine Number:** Used to screen for atypical pseudocholinesterase; a low number indicates an increased risk of prolonged apnea after Succinylcholine administration.

Q: Children are more susceptible than adults to which of the following actions of atropine?

Central excitant and hyperthermic effect. **Explanation:** **1. Why Option C is Correct:** Children, especially infants, exhibit a heightened sensitivity to the central nervous system (CNS) and thermoregulatory effects of atropine. This is primarily due to an **immature blood-brain barrier** and a highly sensitive hypothalamic thermoregulatory center. Atropine causes "Atropine Fever" by two mechanisms: * **Central Effect:** Stimulation of the higher centers leading to restlessness, confusion, and delirium (Central Excitant). * **Peripheral Effect:** Suppression of sweat gland activity (eccrine glands), which prevents evaporative cooling. In children, even modest doses can lead to a dangerous rise in body temperature (Hyperthermia). **2. Why Other Options are Incorrect:** * **A & B (Cycloplegic and Gastric effects):** While atropine does cause pupillary dilation and reduced gastric secretions in children, their sensitivity to these effects is comparable to adults. They are not "more susceptible" to these specific pharmacological actions relative to the life-threatening hyperthermic response. * **D (Tachycardia):** Interestingly, in very young infants, the tachycardic response to atropine may actually be *less* pronounced because their resting vagal tone is lower than that of adults. **3. High-Yield Clinical Pearls for NEET-PG:** * **Atropine Poisoning Mnemonic:** "Hot as a hare (hyperthermia), Red as a beet (flushing), Dry as a bone (anhidrosis), Blind as a bat (cycloplegia), Mad as a hatter (delirium)." * **Antidote:** **Physostigmine** (a tertiary amine that crosses the BBB) is the drug of choice for central anticholinergic toxicity. * **Contraindication:** Atropine is strictly contraindicated in children with high fever, as it can exacerbate hyperpyrexia to fatal levels.

Q: Clonidine is used for which of the following conditions?

All of the above. **Explanation:** Clonidine is a centrally acting **$\alpha_2$-selective adrenergic agonist**. By stimulating presynaptic $\alpha_2$ receptors in the brainstem, it decreases sympathetic outflow, leading to a variety of clinical applications beyond its traditional role as an antihypertensive. **Breakdown of Indications:** * **Opioid Withdrawal Syndrome:** This is a high-yield indication. Opioid withdrawal causes a "sympathetic storm" (tachycardia, hypertension, tremors). Clonidine suppresses this autonomic overactivity by reducing norepinephrine release from the locus coeruleus. * **Diabetic Diarrhea:** In diabetic autonomic neuropathy, loss of $\alpha_2$ adrenergic control in the gut leads to secretomotor instability. Clonidine stimulates $\alpha_2$ receptors on enterocytes, increasing water and electrolyte absorption and decreasing intestinal secretion. * **Migraine Prophylaxis:** Clonidine is used as a prophylactic agent (though not first-line) because it reduces vascular reactivity and prevents the excessive fluctuations in cranial vessel diameter associated with migraine attacks. **Why "All of the Above" is Correct:** Since Clonidine effectively modulates the sympathetic nervous system and peripheral secretomotor functions, it is clinically utilized in all three scenarios mentioned. **NEET-PG High-Yield Pearls:** * **Mechanism:** $\alpha_2$ agonist $\rightarrow$ $\downarrow$ cAMP $\rightarrow$ $\downarrow$ Sympathetic outflow. * **Other Uses:** ADHD (non-stimulant), Tourette syndrome, and as an adjuvant in spinal anesthesia to prolong analgesia. * **Side Effects:** Sedation, xerostomia (dry mouth), and **Rebound Hypertension** (if stopped abruptly due to up-regulation of receptors). * **Diagnostic Test:** The "Clonidine Suppression Test" is used to diagnose Pheochromocytoma (fails to suppress catecholamines in positive cases).

Q: Which of the following drugs used in Benign Prostatic Hyperplasia (BHP) has a negligible risk of causing postural hypotension?

Silodosin. ### Explanation The correct answer is **Silodosin**. **1. Why Silodosin is correct:** The primary goal in treating Benign Prostatic Hyperplasia (BPH) is to relax the smooth muscles of the prostate and bladder neck. This is mediated by **$\alpha_{1A}$-adrenergic receptors**. However, blood pressure regulation is primarily mediated by **$\alpha_{1B}$-adrenergic receptors** located in the vascular smooth muscle. Silodosin (along with Tamsulosin) is a **highly selective $\alpha_{1A}$ blocker** [2]. Because it has minimal affinity for $\alpha_{1B}$ receptors, it does not cause significant peripheral vasodilation [1]. Consequently, it effectively relieves urinary symptoms with a **negligible risk of postural (orthostatic) hypotension**, making it safer for elderly patients or those already on antihypertensives [1], [2]. **2. Why the other options are incorrect:** * **Prazosin, Terazosin, and Doxazosin:** These are **non-selective $\alpha_1$ blockers**. They block both $\alpha_{1A}$ (prostate) and $\alpha_{1B}$ (blood vessels) receptors. While effective for BPH, their potent vasodilatory effect frequently leads to "first-dose phenomenon," syncope, and significant postural hypotension [1]. **3. Clinical Pearls for NEET-PG:** * **Uro-selectivity:** Silodosin > Tamsulosin > Alfuzosin. * **Side Effect Profile:** While Silodosin has the least cardiovascular risk, it has the highest incidence of **retrograde ejaculation** (due to $\alpha_{1A}$ blockade in the vas deferens) [1]. * **IFIS:** All $\alpha_1$ blockers are associated with **Intraoperative Floppy Iris Syndrome (IFIS)** during cataract surgery; the drug should be stopped prior to the procedure. * **Long-acting agents:** Doxazosin and Terazosin have longer half-lives compared to Prazosin and are also used in the management of hypertension.

Question 1

What is true about phenylephrine?

Accepted Answer

Increases peripheral resistance

Answer

Causes hypotension

Answer

Causes cardiac depression

Answer

Increases intraocular pressure by constricting the ciliary body blood vessels

Question 2

Exocytic release of acetylcholine (ACH) is blocked by which of the following?

Accepted Answer

Botulinum toxin

Answer

Hemicholium

Answer

Alphabungarotoxin

Answer

Vesamicol

Question 3

Which of the following is the earliest and shortest-acting skeletal muscle relaxant?

Accepted Answer

Suxamethonium

Answer

Rocuronium

Answer

Vecuronium

Answer

Atracurium

Question 4

Which drug is used for differentiating myasthenia gravis from cholinergic crisis?

Accepted Answer

Edrophonium

Answer

Neostigmine

Answer

Atropine

Answer

Acetylcholine

Question 5

Which of the following is an ultrashort acting muscle relaxant?

Accepted Answer

Succinyl choline

Answer

Rocuronium

Answer

Atracurium

Answer

Doxacurium

Question 6

Children are more susceptible than adults to which of the following actions of atropine?

Accepted Answer

Central excitant and hyperthermic effect

Answer

Cycloplegic effect

Answer

Gastric antisecretory effect

Answer

Tachycardia

Question 7

Metyrosine is indicated in which of the following conditions?

Accepted Answer

Malignant pheochromocytoma

Answer

Addison's disease

Answer

Depression

Answer

Hypertensive emergency

Question 8

Which of the following is NOT a feature of metoprolol when compared to propranolol?

Accepted Answer

Less likely to cause bradycardia

Answer

Ineffective in suppressing muscle tremor

Answer

Safer in diabetics

Answer

Less likely to worsen Raynaud's disease

Question 9

Clonidine is used for which of the following conditions?

Accepted Answer

All of the above

Answer

Migraine

Answer

Opioid withdrawal syndrome

Answer

Diabetic diarrhea

Question 10

Which of the following drugs used in Benign Prostatic Hyperplasia (BHP) has a negligible risk of causing postural hypotension?

Accepted Answer

Silodosin

Answer

Doxazosin

Answer

Terazosin

Answer

Prazosin

Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs — MCQs

On this page

Practice by Chapter

Want unlimited practice?