Autonomic Nervous System Drugs Practice Questions

Q: Which of the following drugs selectively acts on beta receptors?

Isoprenaline. The core concept tested here is the **receptor selectivity** of sympathomimetic amines. **Isoprenaline (Isoproterenol)** is a potent, non-selective **pure beta-adrenergic agonist**. It acts on both $\beta_1$ (heart) and $\beta_2$ (smooth muscle) receptors with negligible activity at $\alpha$ receptors. This results in increased cardiac output (positive inotropy and chronotropy) and marked peripheral vasodilation (decreased peripheral resistance). **Analysis of Incorrect Options:** * **Dopamine (A):** Shows dose-dependent selectivity. At low doses, it acts on $D_1$ receptors; at medium doses, on $\beta_1$; and at high doses, it acts on $\alpha_1$ receptors. * **Noradrenaline (C):** Primarily acts on $\alpha_1$, $\alpha_2$, and $\beta_1$ receptors. It has **minimal to no effect on $\beta_2$ receptors**, which is why it causes intense vasoconstriction. * **Adrenaline (D):** A potent agonist at **both $\alpha$ and $\beta$ receptors** ($\alpha_1, \alpha_2, \beta_1, \beta_2$). While it has high affinity for $\beta$ receptors at low doses, it is not "selective" for them as it recruits $\alpha$ effects at higher concentrations. **High-Yield Clinical Pearls for NEET-PG:** * **Isoprenaline** is the drug of choice for **Torsades de Pointes** (if magnesium fails) and is used in complete heart block. * **Adrenaline** is the drug of choice for **Anaphylactic Shock** (1:1000 IM). * **Noradrenaline** is the drug of choice for **Septic Shock**. * **Dobutamine** is a relatively selective $\beta_1$ agonist used in cardiogenic shock.

Q: Which of the following is a spasmolytic analgesic?

Dicyclomine. ### 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).

Q: Which of the following is an adverse effect of 2 agonists?

Hypokalemia. ### Explanation **Mechanism of Action for Hypokalemia:** $\beta_2$ agonists (like Salbutamol or Terbutaline) stimulate $\beta_2$ receptors, which activates the **Na⁺/K⁺-ATPase pump** located on the cell membranes of skeletal muscles and the liver. This activation causes an influx of potassium ions from the extracellular fluid (plasma) into the intracellular compartment. This shift results in a decrease in serum potassium levels (**Hypokalemia**). This effect is dose-dependent and is often utilized therapeutically in the emergency management of hyperkalemia. **Analysis of Incorrect Options:** * **A. Hypoglycemia:** Incorrect. $\beta_2$ stimulation actually promotes glycogenolysis in the liver and skeletal muscle, leading to **hyperglycemia**, not hypoglycemia. * **B. Hypomagnesemia:** While $\beta_2$ agonists can cause a mild decrease in magnesium, it is not the classic or most frequently tested metabolic side effect compared to hypokalemia. * **C. Hypophosphatemia:** While intracellular shifts can occasionally affect phosphate, it is not a primary or characteristic adverse effect of $\beta_2$ agonists. **High-Yield Clinical Pearls for NEET-PG:** * **Metabolic Effects:** $\beta_2$ agonists cause "Hypo-K" (Hypokalemia) and "Hyper-G" (Hyperglycemia). * **Cardiac Effects:** They cause tachycardia (via direct $\beta_2$ stimulation on the heart and reflex tachycardia due to peripheral vasodilation). * **Muscle Effects:** Fine skeletal muscle **tremors** are the most common side effect (due to $\beta_2$ receptors on the muscle spindles). * **Tolerance:** Prolonged use leads to **downregulation (tachyphylaxis)** of $\beta_2$ receptors, which can be prevented by co-administration of corticosteroids.

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

Agonist of D1 and D2 receptors. **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.

Q: Action of which of the following drugs is rapidly terminated?

Succinylcholine. **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).

Q: All of the following are adverse effects of atropine, except:

Bronchoconstriction. **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).

Q: Which of the following medications is typically avoided in patients with pheochromocytoma due to the risk of hypertensive crisis?

Metoclopramide. In patients with **Pheochromocytoma**, certain medications can trigger a life-threatening hypertensive crisis by stimulating the release of catecholamines from the tumor or blocking their reuptake [1]. **Why Metoclopramide is the Correct Answer:** Metoclopramide is a dopamine ($D_2$) receptor antagonist. In the adrenal medulla, dopamine normally exerts an inhibitory effect on catecholamine release. By blocking these inhibitory receptors, metoclopramide causes a sudden, massive release of stored epinephrine and norepinephrine from the pheochromocytoma. Additionally, it may directly stimulate the tumor. This leads to a precipitous rise in blood pressure, making it strictly contraindicated in these patients. **Analysis of Incorrect Options:** * **A. Promethazine:** While it is an antihistamine with some sedative properties, it does not typically trigger catecholamine release. However, phenothiazines as a class are generally used with caution. * **C. Haloperidol:** Although an antipsychotic, it is not the classic trigger associated with hypertensive crisis in this context compared to metoclopramide. * **D. Ondansetron:** This is a $5-HT_3$ receptor antagonist and is considered a safe antiemetic choice for patients with pheochromocytoma. **High-Yield Clinical Pearls for NEET-PG:** * **Drugs to Avoid in Pheochromocytoma:** Metoclopramide, Tricyclic Antidepressants (TCAs), Glucagon (used in provocative tests, now obsolete), Beta-blockers (if given without prior Alpha-blockade), and certain anesthetics like Desflurane. * **The "Rule of 10s":** 10% bilateral, 10% malignant, 10% pediatric, 10% extra-adrenal (Paraganglioma), and 10% familial [1]. * **Management Sequence:** Always **Alpha-blockade first** (e.g., Phenoxybenzamine) for 10–14 days, followed by Beta-blockade to prevent "unopposed alpha stimulation" which causes severe hypertension [1].

Question 1

Which of the following is a uroselective a1A blocker?

Accepted Answer

Tamsulosin

Answer

Prazosin

Answer

Urapidil

Answer

Terazosin

Question 2

Which of the following drugs selectively acts on beta receptors?

Accepted Answer

Isoprenaline

Answer

Dopamine

Answer

Noradrenaline

Answer

Adrenaline

Question 3

Which of the following is a spasmolytic analgesic?

Accepted Answer

Dicyclomine

Answer

Physostigmine

Answer

Tropicamide

Answer

None of the above

Question 4

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

Accepted Answer

Urinary symptoms of benign prostatic hyperplasia (BHP)

Answer

Tachycardia

Answer

Bronchial dilatation

Answer

Migraine

Question 5

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

Accepted Answer

Prazosin

Answer

Finasteride

Answer

Goserelin

Answer

Amphetamine

Question 6

Which of the following is an adverse effect of 2 agonists?

Accepted Answer

Hypokalemia

Answer

Hypoglycemia

Answer

Hypomagnesemia

Answer

Hypophosphatemia

Question 7

Which of the following statements is not true regarding Dobutamine?

Accepted Answer

Agonist of D1 and D2 receptors

Answer

Derivative of dopamine

Answer

Selective beta-agonistic action

Answer

Reduced chances of arrhythmia than adrenaline

Question 8

Action of which of the following drugs is rapidly terminated?

Accepted Answer

Succinylcholine

Answer

Atracurium

Answer

Pancuronium

Answer

D-tubocurarine

Question 9

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

Accepted Answer

Bronchoconstriction

Answer

Tachycardia

Answer

Dry mouth

Answer

Mydriasis

Question 10

Which of the following medications is typically avoided in patients with pheochromocytoma due to the risk of hypertensive crisis?

Accepted Answer

Metoclopramide

Answer

Promethazine

Answer

Haloperidol

Answer

Ondansetron

Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs — MCQs

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