Central Nervous System Pharmacology Practice Questions

Q: Which of the following local anesthetics can cause methemoglobinemia?

All of the above. **Explanation:** Methemoglobinemia is a clinical condition where the ferrous iron ($Fe^{2+}$) in hemoglobin is oxidized to the ferric state ($Fe^{3+}$), which cannot bind oxygen and causes a "left shift" in the dissociation curve, leading to tissue hypoxia. **Why "All of the Above" is correct:** While **Prilocaine** is the most classic association taught for exams, several local anesthetics (LAs) are capable of inducing methemoglobinemia through their metabolites: * **Prilocaine:** Its metabolite, **o-toluidine**, is a potent oxidizing agent. This is the most common cause in clinical practice. * **Benzocaine:** Frequently used in topical sprays (for endoscopy or dental procedures), it is a well-documented cause of sudden, severe methemoglobinemia. * **Lignocaine (Lidocaine):** Although much rarer than the others, lidocaine is metabolized to monoethylglycinexylidide, which can contribute to oxidative stress and methemoglobin formation, especially in high doses or susceptible individuals. **Clinical Pearls for NEET-PG:** * **Classic Presentation:** A patient undergoing a procedure (like bronchoscopy or spinal anesthesia) develops "chocolate-colored blood" and cyanosis that does not improve with 100% oxygen. * **Diagnosis:** A "saturation gap" is observed (low $SpO_2$ on pulse oximetry but normal $PaO_2$ on ABG). * **Drug of Choice:** **Methylene Blue** (1–2 mg/kg IV over 5 minutes). It acts as an electron donor to reduce ferric iron back to ferrous iron. * **Contraindication:** Avoid Methylene Blue in patients with **G6PD deficiency**, as it can precipitate hemolysis. Use Vitamin C (Ascorbic acid) as an alternative. * **Other causative agents:** Nitrites, Nitroglycerin, Sulfonamides, and Dapsone.

Q: Which agent is often combined with local anesthetics to prevent its systemic distribution from the site of injection?

Epinephrine. **Explanation:** The correct answer is **Epinephrine (Adrenaline)**. **Mechanism of Action:** Local anesthetics (LAs) are vasodilators (except cocaine). When injected, they increase local blood flow, which promotes rapid systemic absorption. To counteract this, a vasoconstrictor like **Epinephrine** is added. By stimulating **$\alpha_1$-adrenergic receptors**, it causes localized vasoconstriction, leading to: 1. **Decreased systemic absorption:** Reducing the risk of systemic toxicity (LAST). 2. **Increased duration of action:** The anesthetic remains at the nerve site for a longer period. 3. **Improved surgical field:** Reduced bleeding due to vasoconstriction. **Why other options are incorrect:** * **Acetylcholine:** A neurotransmitter of the parasympathetic system; it generally causes vasodilation and would accelerate systemic distribution. * **Dopamine:** Primarily used as an inotrope/vasopressor in shock; it is not used locally with LAs. * **GABA:** The primary inhibitory neurotransmitter in the CNS; it has no peripheral vasoconstrictive properties. **High-Yield Clinical Pearls for NEET-PG:** * **Standard Concentration:** Epinephrine is typically added in a concentration of **1:200,000** (5 µg/mL). * **Contraindications:** Avoid using Epinephrine-containing LAs in areas supplied by **end-arteries** (e.g., fingers, toes, tip of the nose, ears, and penis) to prevent ischemic necrosis/gangrene. * **pH Effect:** Epinephrine is acidic; adding it to LA can lower the pH, potentially slowing the onset of block unless buffered with sodium bicarbonate. * **Felypressin:** Another vasoconstrictor (synthetic vasopressin) sometimes used in dental anesthesia, especially when Epinephrine is contraindicated (e.g., hyperthyroidism).

Q: Which local anesthetic has the maximum ionized form at physiological pH?

Chloroprocaine. **Explanation:** The degree of ionization of a local anesthetic (LA) is determined by its **pKa** (the pH at which 50% of the drug exists in ionized and 50% in unionized form). Local anesthetics are weak bases. According to the Henderson-Hasselbalch equation, the closer the pKa of a drug is to the physiological pH (7.4), the greater the fraction of the drug that remains in the **unionized (lipid-soluble)** form. Conversely, a higher pKa results in a greater fraction of the **ionized (water-soluble)** form at pH 7.4. **Why Chloroprocaine is correct:** * **Chloroprocaine** has the highest pKa among the options (approximately **8.7 to 9.1**). * Because its pKa is furthest from 7.4, it exists predominantly in the ionized form (approx. 95%+) at physiological pH. This explains its **slow onset of action** in vitro, although clinically, this is often compensated for by using higher concentrations (3%). **Analysis of Incorrect Options:** * **Lignocaine (pKa ~7.7-7.8):** Its pKa is very close to 7.4, meaning a significant portion (about 25%) remains unionized, leading to a **fast onset**. * **Etidocaine (pKa ~7.7):** Similar to Lignocaine, it has a low pKa and exists largely in the unionized form. * **Bupivacaine (pKa ~8.1):** While higher than Lignocaine, it is still lower than Chloroprocaine, meaning it has less ionized fraction than Chloroprocaine. **High-Yield Clinical Pearls for NEET-PG:** 1. **Onset of Action:** Determined by **pKa**. Lower pKa = Faster onset (Exception: Chloroprocaine is clinically fast due to high concentration). 2. **Potency:** Determined by **Lipid Solubility**. 3. **Duration of Action:** Determined by **Protein Binding**. 4. **Infected Tissue:** Acidic environments (low pH) increase the ionized fraction of LAs, which is why they work poorly in abscesses or inflamed tissues. 5. **Chloroprocaine** is the least toxic LA because it is rapidly metabolized by plasma pseudocholinesterase.

Q: Which of the following is not an analeptic agent?

Doxacurium. **Explanation:** **Analeptics** (Respiratory Stimulants) are drugs used to stimulate the central nervous system, particularly the respiratory and vasomotor centers in the medulla. They were historically used to treat acute respiratory failure or drug-induced CNS depression. **1. Why Doxacurium is the Correct Answer:** **Doxacurium** is a **long-acting, non-depolarizing neuromuscular blocking agent** (skeletal muscle relaxant) belonging to the benzylisoquinolinium class. Unlike analeptics, which stimulate the CNS, Doxacurium acts peripherally at the nicotinic receptors of the motor endplate to cause muscle paralysis. It has no respiratory stimulant properties; in fact, it causes respiratory paralysis as a side effect of its primary action. **2. Analysis of Incorrect Options:** * **Doxapram (Option A):** This is the most commonly used analeptic. It acts by stimulating peripheral chemoreceptors and, at higher doses, the central respiratory centers. It is used for post-operative respiratory depression. * **Nikethamide (Option B):** A traditional respiratory stimulant that acts directly on the medulla to increase the rate and depth of respiration. * **Propylbucamide (Option D):** Also known as Vanylamide, it is a respiratory stimulant occasionally mentioned in older pharmacological literature as an analeptic. **High-Yield Clinical Pearls for NEET-PG:** * **Doxapram** is the drug of choice for treating respiratory depression caused by COPD or anesthesia, but it has a narrow therapeutic index. * **Doxacurium** is notable for being one of the most potent neuromuscular blockers with minimal cardiovascular side effects (no histamine release). * **Analeptics** are largely replaced today by mechanical ventilation and specific antidotes (e.g., Naloxone for opioids, Flumazenil for benzodiazepines) due to their risk of inducing convulsions.

Q: Which of the following is true regarding Zolpidem?

Sedation is less than Diazepam. Zolpidem belongs to the "Z-drugs" (Non-benzodiazepine hypnotics), which are chemically distinct from benzodiazepines (BZDs) but share a similar mechanism of action. [1] **1. Why Option C is Correct:** Zolpidem is highly selective for the **$\alpha_1$ subunit** of the $GABA_A$ receptor (BZ1 receptor). [4] This subunit primarily mediates sedation and hypnosis. Unlike Diazepam, which acts non-selectively on $\alpha_1, \alpha_2, \alpha_3,$ and $\alpha_5$ subunits, Zolpidem lacks significant anticonvulsant, muscle relaxant, and anxiolytic properties. [1] Consequently, while it is an effective hypnotic, the overall CNS depressant effect and "heaviness" of sedation are considered less than that of long-acting BZDs like Diazepam. It also causes minimal disruption to the sleep architecture (less suppression of REM sleep). **2. Why Other Options are Incorrect:** * **Option A:** Zolpidem is **selective for BZ1 ( $\alpha_1$) receptors** only. It has negligible affinity for BZ2 receptors ($\alpha_2, \alpha_3, \alpha_5$), which are responsible for anxiolysis and muscle relaxation. [4] * **Option B:** Since Zolpidem binds to the same BZD-binding site on the $GABA_A$ receptor (albeit selectively), its actions **are rapidly reversed by Flumazenil** (a competitive BZD antagonist). [3] **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Zolpidem is often preferred for the short-term treatment of **insomnia** (specifically sleep onset) due to its short half-life (~2 hours) and lack of "hangover" effects. * **Safety Profile:** It has a lower potential for tolerance and dependence compared to traditional BZDs. [1] * **Side Effects:** Can cause residual daytime somnolence and, rarely, complex sleep behaviors (sleep-walking/driving). * **Other Z-drugs:** Zaleplon (shortest acting) and Eszopiclone (longest acting, used for chronic insomnia). [2]

Q: A 72-year-old patient with Parkinsonism presents with swollen feet. The feet are red, tender, and very painful. Which medication, if stopped, would likely resolve these symptoms within a few days?

Amantadine. ### Explanation The patient is presenting with **Livedo Reticularis** and **Ankle Edema**, which are classic side effects associated with **Amantadine** therapy. **1. Why Amantadine is Correct:** Amantadine is an antiviral drug used in Parkinsonism to increase dopamine release and inhibit reuptake. A unique side effect of Amantadine is **Livedo Reticularis**—a purplish, lace-like discoloration of the skin, often accompanied by **peripheral edema** (swollen, red, and painful feet). This occurs due to the depletion of catecholamines in peripheral nerve terminals, leading to local vasoconstriction and fluid extravasation. These symptoms are reversible and typically resolve within a few days to weeks after discontinuing the drug. **2. Why Incorrect Options are Wrong:** * **Benztropine:** An anticholinergic drug. Common side effects include "dry" symptoms (dry mouth, blurred vision, urinary retention, and constipation), not peripheral edema or skin discoloration. * **Bromocriptine:** An ergot-derived dopamine agonist. While it can cause erythromelalgia (red, painful extremities), it is more famously associated with **pulmonary/retroperitoneal fibrosis**. Amantadine is the more "textbook" cause for the specific presentation of livedo reticularis in NEET-PG questions. * **Levodopa:** The gold standard for Parkinson’s. Its primary side effects are GI upset (nausea/vomiting), dyskinesias, and "on-off" phenomena, but it does not typically cause localized pedal edema or livedo reticularis. **Clinical Pearls for NEET-PG:** * **Amantadine Triple Action:** Increases dopamine release, blocks dopamine reuptake, and acts as an **NMDA receptor antagonist**. * **High-Yield Side Effect:** Always associate "Livedo Reticularis" with Amantadine in the context of Parkinson’s. * **Contraindication:** Use with caution in patients with a history of heart failure, as the edema can exacerbate the condition.

Q: Tetrabenazine acts by inhibiting which of the following?

Vesicular monoamine transporter. **Explanation:** **Tetrabenazine** is a reversible inhibitor of the **Vesicular Monoamine Transporter type 2 (VMAT2)**. 1. **Mechanism of Action (Why B is correct):** VMAT2 is responsible for transporting monoamines (Dopamine, Serotonin, Norepinephrine) from the cytosol into synaptic vesicles for storage. By inhibiting VMAT2, Tetrabenazine prevents the packaging of these neurotransmitters, leading to their degradation by cytosolic MAO. This results in the **depletion of monoamine stores**, particularly dopamine, in the nerve terminals. This reduction in dopamine levels is clinically useful in treating hyperkinetic movement disorders. 2. **Why other options are incorrect:** * **MAO-B (A):** Inhibitors like Selegiline or Rasagiline prevent the breakdown of dopamine; Tetrabenazine actually promotes its depletion. * **Dopamine D2 receptors (C):** While Tetrabenazine has a weak affinity for D2 receptors, its primary and potent mechanism is VMAT2 inhibition. Direct D2 blockers are typical/atypical antipsychotics (e.g., Haloperidol). * **COMT (D):** COMT inhibitors (e.g., Entacapone) are used in Parkinson’s disease to prevent the peripheral breakdown of Levodopa. **Clinical Pearls for NEET-PG:** * **Indications:** Primarily used for **Huntington’s Chorea** and other hyperkinetic disorders like Tardive Dyskinesia or Tourette syndrome. * **Side Effects:** Due to monoamine depletion, it can cause **depression**, sedation, and secondary Parkinsonism. * **Deutetrabenazine:** A newer, deuterated form of Tetrabenazine with a longer half-life and better tolerability. * **Valbenazine:** A highly selective VMAT2 inhibitor specifically approved for Tardive Dyskinesia.

Q: Which local anesthetic can cause Methaemoglobinemia?

Prilocaine. ### Explanation **Correct Answer: B. Prilocaine** **1. Why Prilocaine is the correct answer:** Prilocaine is an amide-type local anesthetic that is metabolized in the liver into **o-toluidine** (ortho-toluidine). This specific metabolite is a potent oxidizing agent that converts ferrous iron ($Fe^{2+}$) in hemoglobin to ferric iron ($Fe^{3+}$), forming **methemoglobin**. Unlike normal hemoglobin, methemoglobin cannot bind oxygen and causes a "left shift" in the dissociation curve, leading to tissue hypoxia and characteristic "chocolate-colored blood" or cyanosis that does not improve with oxygen. **2. Analysis of incorrect options:** * **A. Procaine:** This is an ester-type local anesthetic. While it is metabolized into para-aminobenzoic acid (PABA)—which is associated with allergic reactions—it does not produce metabolites that cause significant oxidative stress to hemoglobin. * **C & D:** Since only Prilocaine (and its metabolite o-toluidine) is classically associated with this side effect among the options provided, these choices are incorrect. **3. Clinical Pearls for NEET-PG:** * **The "Other" Culprit:** Apart from Prilocaine, **Benzocaine** (often used as a topical spray for endoscopy) is the other major local anesthetic frequently implicated in causing methemoglobinemia. * **Threshold:** Methemoglobinemia typically occurs when the dose of Prilocaine exceeds **600 mg**. * **Antidote:** The definitive treatment is **Methylene Blue** (1–2 mg/kg IV over 5 minutes). It acts as a cofactor for the enzyme NADPH-methemoglobin reductase to reduce ferric iron back to its functional ferrous state. * **High-Yield Association:** Patients with **G6PD deficiency** are at a higher risk and should be treated cautiously, as Methylene Blue may be ineffective or cause hemolysis in these individuals.

Q: The selective MAO-B inhibitor among the following is

Selegiline. **Explanation:** Monoamine oxidase (MAO) is an enzyme responsible for the oxidative deamination of biogenic amines. It exists in two isoforms: **MAO-A** (preferentially degrades serotonin, norepinephrine, and dopamine) and **MAO-B** (preferentially degrades dopamine). **1. Why Selegiline is Correct:** **Selegiline** is a selective, irreversible inhibitor of **MAO-B**. By inhibiting the breakdown of dopamine in the striatum, it increases dopamine levels without significantly affecting systemic norepinephrine or serotonin. This selectivity makes it highly effective in the management of **Parkinson’s disease** (often as an adjunct to Levodopa) and, at higher doses, for depression. **2. Analysis of Incorrect Options:** * **Clorgyline:** This is a selective and irreversible inhibitor of **MAO-A**. It is primarily used in research and is not a standard clinical treatment for Parkinson's. * **Moclobemide:** This is a **RIMA** (Reversible Inhibitor of MAO-A). It is used as an antidepressant and is safer than older MAOIs because it carries a lower risk of the "cheese reaction." * **Tranylcypromine:** This is a **non-selective**, irreversible inhibitor of both MAO-A and MAO-B. It is an older antidepressant associated with significant dietary restrictions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cheese Reaction (Hypertensive Crisis):** Occurs when patients on non-selective MAOIs or MAO-A inhibitors consume tyramine-rich food (aged cheese, wine). Tyramine displaces NE, leading to massive sympathetic discharge. * **Selectivity Loss:** Selegiline loses its MAO-B selectivity at higher doses (>10 mg/day), increasing the risk of hypertensive crisis. * **Rasagiline:** Another potent, irreversible MAO-B inhibitor used in Parkinson’s, often preferred over Selegiline as it is not metabolized into amphetamine derivatives. * **Drug Interaction:** Never combine MAOIs with SSRIs/SNRIs due to the risk of **Serotonin Syndrome**.

Question 1

Which of the following local anesthetics can cause methemoglobinemia?

Accepted Answer

All of the above

Answer

Prilocaine

Answer

Lignocaine

Answer

Benzocaine

Question 2

Which agent is often combined with local anesthetics to prevent its systemic distribution from the site of injection?

Accepted Answer

Epinephrine

Answer

Acetylcholine

Answer

Dopamine

Answer

g-Aminobutyric acid (GABA)

Question 3

In a patient receiving a ketogenic diet for epilepsy, which one of the following antiepileptic agents should be avoided?

Accepted Answer

Sodium valproate

Answer

Phenytoin

Answer

Carbamazepine

Answer

Lamotrigine

Question 4

Which local anesthetic has the maximum ionized form at physiological pH?

Accepted Answer

Chloroprocaine

Answer

Lignocaine

Answer

Etidocaine

Answer

Bupivacaine

Question 5

Which of the following is not an analeptic agent?

Accepted Answer

Doxacurium

Answer

Doxapram

Answer

Nikethamide

Answer

Propylbucamide

Question 6

Which of the following is true regarding Zolpidem?

Accepted Answer

Sedation is less than Diazepam

Answer

Acts on Benzodiazepine receptor 1 and 2

Answer

Action is not reversed by flumazenil

Answer

All of the above

Question 7

A 72-year-old patient with Parkinsonism presents with swollen feet. The feet are red, tender, and very painful. Which medication, if stopped, would likely resolve these symptoms within a few days?

Accepted Answer

Amantadine

Answer

Benztropine

Answer

Bromocriptine

Answer

Levodopa

Question 8

Tetrabenazine acts by inhibiting which of the following?

Accepted Answer

Vesicular monoamine transporter

Answer

MAO-B

Answer

Dopamine D2 receptors

Answer

COMT

Question 9

Which local anesthetic can cause Methaemoglobinemia?

Accepted Answer

Prilocaine

Answer

Procaine

Answer

Both Procaine and Prilocaine

Answer

None of the above

Question 10

The selective MAO-B inhibitor among the following is

Accepted Answer

Selegiline

Answer

Clorgyline

Answer

Moclobemide

Answer

Tranylcypromine

Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology — MCQs

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