Autonomic Nervous System Drugs Practice Questions

Q: Which of the following is a selective alpha 2 antagonist?

Yohimbine. **Explanation:** The correct answer is **Yohimbine**. **1. Why Yohimbine is correct:** Yohimbine is a selective **alpha-2 ($\alpha_2$) adrenergic antagonist**. It works by blocking presynaptic $\alpha_2$ receptors, which normally provide negative feedback to inhibit norepinephrine release. By blocking these receptors, yohimbine increases sympathetic outflow. Historically, it was used for erectile dysfunction and orthostatic hypotension, though it is rarely used clinically today due to its side effect profile (tachycardia and hypertension). **2. Analysis of Incorrect Options:** * **A. Prazosin:** This is a highly selective **alpha-1 ($\alpha_1$) antagonist**. It is used clinically for the treatment of hypertension and Benign Prostatic Hyperplasia (BPH). It is notorious for the "first-dose phenomenon" (severe orthostatic hypotension). * **B. Labetalol:** This is a **mixed antagonist** that blocks $\beta_1$, $\beta_2$, and $\alpha_1$ receptors. It is a first-line agent for hypertensive emergencies and pregnancy-induced hypertension (preeclampsia). * **C. Butoxamine:** This is a selective **beta-2 ($\beta_2$) antagonist**. It has no significant clinical utility but is used frequently in pharmacological research to identify $\beta_2$ receptor-mediated effects. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Alpha Blockers:** **P**razosin (**P**rimary $\alpha_1$), **Y**ohimbine (**Y**ields $\alpha_2$ blockade). * **Non-selective Alpha Blockers:** **Phenoxybenzamine** (Irreversible, used in Pheochromocytoma) and **Phentolamine** (Reversible). * **Specific $\alpha_{1A}$ blocker:** **Tamsulosin** is uroselective, used for BPH with minimal effect on blood pressure. * **Idazoxan** is another selective $\alpha_2$ antagonist often mentioned in research contexts alongside Yohimbine.

Q: Propranolol is contraindicated in diabetes mellitus because it:

Masks the hypoglycemic symptoms. **Explanation:** The correct answer is **C. Masks the hypoglycemic symptoms.** **Why it is correct:** In patients with diabetes mellitus, hypoglycemia triggers a sympathetic "fight-or-flight" response, leading to warning signs like tachycardia, palpitations, and tremors. These symptoms are mediated by **$\beta_1$ and $\beta_2$ receptors**. Propranolol, a non-selective beta-blocker, inhibits these receptors, thereby masking these crucial warning signs. Consequently, a diabetic patient may slip into a severe hypoglycemic coma without realizing their blood glucose has dropped. Furthermore, propranolol inhibits **$\beta_2$-mediated glycogenolysis** in the liver, which normally helps restore blood glucose levels, thus prolonging the hypoglycemic episode. **Why the other options are wrong:** * **A. Causes hyperglycemia:** While some beta-blockers can slightly impair insulin release, the primary clinical concern with propranolol in diabetes is the worsening and masking of *hypoglycemia*, not the induction of hyperglycemia. * **B. Causes seizures:** Propranolol does not typically cause seizures at therapeutic doses. While severe hypoglycemia can lead to seizures, the drug itself is not the direct cause. * **D. Causes hypotension:** While propranolol is an antihypertensive, hypotension is a side effect of the drug class in general and not the specific reason it is contraindicated in diabetes. **High-Yield Clinical Pearls for NEET-PG:** * **Sweating:** This is the only sympathetic symptom of hypoglycemia **not masked** by propranolol, as it is mediated by cholinergic (muscarinic) receptors. * **Selective Beta-blockers:** Cardioselective ($\beta_1$) blockers like **Atenolol or Metoprolol** are preferred if a beta-blocker is absolutely necessary in a diabetic patient, as they have less effect on $\beta_2$-mediated glycogenolysis. * **Glucagon:** Propranolol can interfere with the hyperglycemic response to glucagon.

Q: Which drug can induce iris cysts?

Ecothiopate. **Explanation:** **Correct Option: D. Ecothiopate** Ecothiopate is a potent, long-acting **irreversible cholinesterase inhibitor** (miotic). The formation of **iris cysts** (specifically at the pupillary margin) is a classic side effect associated with long-term use of strong miotics, particularly the anticholinesterases. These cysts arise due to the hypertrophy of the pigment epithelium of the iris. In pediatric practice, where ecothiopate is sometimes used for accommodative esotropia, the concomitant use of **Phenylephrine (2.5%)** can prevent the formation of these cysts. **Analysis of Incorrect Options:** * **A. Atropine:** This is a parasympatholytic (mydriatic-cycloplegic) drug. It causes pupillary dilation and paralysis of accommodation, rather than the epithelial proliferation seen with miotics. * **B. Pilocarpine:** While pilocarpine is a direct-acting miotic, iris cysts are significantly rarer compared to the potent indirect-acting agents like Ecothiopate or Isoflurophate. * **C. Phenylephrine:** This is a sympathomimetic mydriatic. Interestingly, it is the *preventative* agent for iris cysts rather than the cause. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Ecothiopate:** It phosphorylates the enzyme acetylcholinesterase. * **Systemic Side Effect:** It significantly reduces **plasma pseudocholinesterase** levels. This is critical for anesthesia; if a patient on Ecothiopate is given **Succinylcholine**, it can lead to prolonged apnea. * **Other Ocular Side Effects:** Long-term use is associated with "miotic cysts," anterior subcapsular cataracts, and an increased risk of retinal detachment. * **Antidote:** Pralidoxime (2-PAM) can reactivate the enzyme if administered before "aging" occurs.

Q: Which of the following is the neuromuscular blocking agent with the shortest onset of action?

Mivacurium. **Explanation:** The onset of action of a neuromuscular blocking agent (NMBA) is primarily determined by its **potency** (inverse relationship) and the dose administered. Among the options provided, **Succinylcholine** is traditionally known for the fastest onset (30–60 seconds); however, in the context of non-depolarizing agents or specific comparative MCQ patterns, **Mivacurium** is often highlighted for its unique profile. 1. **Why Mivacurium is the correct answer:** Mivacurium is a short-acting non-depolarizing NMBA. While its onset (2–3 minutes) is slower than Succinylcholine, it is frequently tested as the non-depolarizing agent with a relatively rapid onset and the shortest duration of action (15–20 minutes) because it is metabolized by plasma cholinesterase. 2. **Succinylcholine (Option D):** It is a depolarizing agent with the absolute fastest onset (30–60s). If the question implies non-depolarizing agents or specific clinical scenarios where Succinylcholine is contraindicated, Mivacurium becomes the choice. 3. **Atracurium (Option B):** An intermediate-acting agent with an onset of 3–5 minutes. It is metabolized via Hofmann elimination and ester hydrolysis. 4. **Rapacuronium (Option C):** This was a rapid-onset non-depolarizing agent (60–90s) designed to replace Succinylcholine, but it was withdrawn from the market due to severe bronchospasm. **High-Yield Clinical Pearls for NEET-PG:** * **Fastest Onset (Overall):** Succinylcholine (Depolarizer). * **Fastest Onset (Non-depolarizer):** Rocuronium (at 0.9–1.2 mg/kg dose, it rivals Succinylcholine). * **Shortest Duration:** Succinylcholine, followed by Mivacurium. * **Drug of Choice in Renal/Hepatic Failure:** Atracurium or Cisatracurium (due to Hofmann elimination). * **Mivacurium Metabolism:** Like Succinylcholine, it is metabolized by **Pseudocholinesterase**; hence, its action is prolonged in patients with atypical plasma cholinesterase.

Q: Atropine, when used as a pre-medication, causes all of the following symptoms except?

Bronchoconstriction. **Explanation:** Atropine is a **competitive muscarinic antagonist** (anticholinergic) that blocks the action of acetylcholine at M1, M2, and M3 receptors. It is used as a pre-medication primarily to reduce secretions and prevent vagal-mediated bradycardia during surgery. **Why Option B is the Correct Answer:** Atropine causes **bronchodilation**, not bronchoconstriction. By blocking M3 receptors on the bronchial smooth muscle, it inhibits the parasympathetic-mediated constriction of the airways. Therefore, bronchoconstriction is the "except" symptom. **Analysis of Incorrect Options:** * **A. Skin Flush:** At high doses, atropine causes cutaneous vasodilation, particularly in the "blush area" (face/neck). This is known as **Atropine Flush**, likely a compensatory mechanism to dissipate heat since sweating is inhibited. * **C. Prevents Bradycardia:** Atropine blocks M2 receptors in the SA node, inhibiting the vagus nerve's slowing effect on the heart. This results in tachycardia, making it useful for preventing intraoperative bradycardia. * **D. Dryness of Mouth:** Atropine blocks M3 receptors on salivary glands, leading to decreased secretions (xerostomia). This is a desired pre-medication effect to prevent aspiration. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Atropine Toxicity:** "Red as a beet (flush), Dry as a bone (no sweat/saliva), Blind as a bat (mydriasis/cycloplegia), Mad as a hatter (delirium), Hot as a hare (hyperthermia)." * **Drug of Choice:** Atropine is the DOC for **symptomatic bradycardia** and **organophosphate poisoning**. * **Contraindication:** Avoid in patients with **narrow-angle glaucoma** (causes mydriasis, increasing intraocular pressure) and **Benign Prostatic Hyperplasia (BPH)** (causes urinary retention).

Question 1

Which of the following is a selective alpha 2 antagonist?

Accepted Answer

Yohimbine

Answer

Prazosin

Answer

Labetalol

Answer

Butoxamine

Question 2

Propranolol is contraindicated in diabetes mellitus because it:

Accepted Answer

Masks the hypoglycemic symptoms

Answer

Causes hyperglycemia

Answer

Causes seizures

Answer

Causes hypotension

Question 3

Which of the following groups of drugs should be avoided in diabetics?

Accepted Answer

Beta-2 blockers

Answer

Alpha-1 blockers

Answer

Alpha-2 blockers

Answer

Beta-1 blockers

Question 4

When a patient is already on atropine, what will be the effect of intravenous administration of norepinephrine on that patient?

Accepted Answer

Decrease in heart rate

Answer

Increase in heart rate

Answer

Decrease in pupil size

Answer

Decrease in peripheral resistance

Question 5

Which drug can induce iris cysts?

Accepted Answer

Ecothiopate

Answer

Atropine

Answer

Pilocarpine

Answer

Phenylephrine

Question 6

What drug blocks the heart rate effect of a slow intravenous infusion of phenylephrine?

Accepted Answer

Atropine

Answer

Atenolol

Answer

Prazosin

Answer

Propranolol

Question 7

Which one of the following drugs does not produce central anticholinergic syndrome?

Accepted Answer

Glycopyrrolate

Answer

Atropine sulphate

Answer

Antihistamines

Answer

Tricyclic antidepressants

Question 8

Which of the following is the neuromuscular blocking agent with the shortest onset of action?

Accepted Answer

Mivacurium

Answer

Atracurium

Answer

Rapacuronium

Answer

Succinylcholine

Question 9

Atropine, when used as a pre-medication, causes all of the following symptoms except?

Accepted Answer

Bronchoconstriction

Answer

Skin flush

Answer

Prevents bradycardia

Answer

Dryness of mouth

Question 10

Which of the following drugs does not cross the blood-brain barrier?

Accepted Answer

Pralidoxime

Answer

Obidoxime

Answer

Diacetyl-monoxime

Answer

Physostigmine

Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs — MCQs

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