Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 661: What is the primary mechanism of action of local anesthetics?

A. Block the release of neurotransmitters
B. Block the influx of sodium into the cell (Correct Answer)
C. Increase the release of inhibitory neurotransmitters
D. Inhibit the efflux of sodium from neurons

Explanation: **Explanation:** Local anesthetics (LAs) act by reversibly binding to the **intracellular portion of voltage-gated sodium channels** in the neuronal membrane. By binding to the inactivated state of these channels, they prevent the **influx of sodium ions**, which is essential for the initiation and propagation of an action potential. Without sodium influx, the threshold for excitation is not reached, leading to a conduction block. **Analysis of Options:** * **Option B (Correct):** LAs block the sodium channel pore from the inside, preventing the rapid depolarization required for nerve impulse transmission. * **Option A:** This is the mechanism of drugs like Botulinum toxin (blocks ACh release) or certain presynaptic modulators, not LAs. * **Option C:** This describes the mechanism of drugs like Benzodiazepines or Barbiturates (enhancing GABAergic activity), which act on the CNS rather than peripheral nerve conduction. * **Option D:** Sodium efflux is primarily managed by the Na+/K+ ATPase pump to restore resting membrane potential; inhibiting this would not cause immediate anesthesia. **High-Yield NEET-PG Pearls:** 1. **State-Dependent Block:** LAs have a higher affinity for channels in the **activated (open)** or **inactivated** states rather than the resting state. This is why rapidly firing nerves are blocked faster (Use-dependent block). 2. **Order of Blockade:** Small, myelinated fibers are blocked first. The sequence is: **Autonomic > Pain > Temperature > Touch > Deep Pressure > Motor.** 3. **pH Sensitivity:** LAs are weak bases. In acidic environments (e.g., infected tissue/pus), they become ionized and cannot cross the lipid membrane, leading to **reduced efficacy.** 4. **Bupivacaine:** Notable for being the most **cardiotoxic** local anesthetic. Intravenous lipid emulsion is the antidote for systemic toxicity.

Question 662: Blockade of nerve conduction by a local anesthetic is characterized by which of the following?

A. Greater potential to block a resting nerve as compared to a stimulated nerve
B. Need to cross the cell membrane to produce the block (Correct Answer)
C. Large myelinated fibers are blocked before small myelinated fibers
D. Causes a consistent change in resting membrane potential

Explanation: **Explanation:** **1. Why Option B is Correct:** Local anesthetics (LAs) are weak bases. To exert their effect, the **unionized (lipid-soluble) form** must cross the neuronal cell membrane. Once inside the axoplasm, the molecule becomes **re-ionized (charged)** due to the lower internal pH. It is this ionized form that binds to the specific receptor site on the **inner (cytoplasmic) surface** of the voltage-gated sodium channel, effectively blocking sodium influx and preventing depolarization. **2. Why the Other Options are Incorrect:** * **Option A:** LAs exhibit **"use-dependent" or "state-dependent" blockade.** They have a higher affinity for channels in the open or inactivated states (which occur during stimulation) rather than the resting state. Thus, a stimulated nerve is blocked faster than a resting one. * **Option C:** Sensitivity to blockade is generally determined by fiber diameter and myelination. **Small myelinated fibers (Aδ)** and **small unmyelinated fibers (C)** are typically blocked before large myelinated fibers (Aα). * **Option D:** LAs do **not** alter the resting membrane potential. They work by decreasing the rate of rise of the action potential and increasing the threshold for electrical excitability until the nerve can no longer propagate an impulse. **High-Yield Clinical Pearls for NEET-PG:** * **Sequence of Blockade:** Pain > Temperature > Touch > Deep Pressure > Motor function. * **pH Effect:** LAs are less effective in **infected/inflamed tissues** because the acidic environment increases ionization outside the cell, preventing the drug from crossing the membrane. * **Sodium Bicarbonate:** Adding $NaHCO_3$ to LAs increases the unionized fraction, speeding up the onset of action. * **Cocaine** is the only LA with intrinsic vasoconstrictive properties; others (like Lidocaine) are vasodilators.

Question 663: Which of the following is NOT a pharmacological action of opioids?

A. Cough suppression
B. Anti-emesis (Correct Answer)
C. Miosis
D. Truncal rigidity

Explanation: **Explanation:** Opioids act primarily on $\mu$, $\kappa$, and $\delta$ receptors in the CNS [2]. Understanding their diverse effects is crucial for NEET-PG. **Why "Anti-emesis" is the correct answer:** Opioids are actually **emetogenic** (pro-emetic), not anti-emetic [3]. They induce nausea and vomiting via two mechanisms: 1. **Direct stimulation** of the Chemoreceptor Trigger Zone (CTZ) in the area postrema of the medulla. 2. **Increasing vestibular sensitivity**, which is why opioid-induced nausea is often worsened by movement [3]. *Note: While high doses may eventually depress the vomiting center, the clinical hallmark is emesis.* **Analysis of Incorrect Options:** * **Cough suppression (Antitussive):** Opioids (like Codeine and Dextromethorphan) suppress the cough reflex by acting directly on the cough center in the medulla [1]. * **Miosis:** Opioids stimulate the **Edinger-Westphal nucleus** of the oculomotor nerve, leading to "pin-point pupils." This is a key diagnostic sign in opioid overdose, as tolerance does not develop to this effect [1]. * **Truncal rigidity:** Rapid intravenous administration of potent opioids (e.g., Fentanyl) can cause supraspinal motor effects leading to "Stiff Man Syndrome" or chest wall rigidity, which can interfere with ventilation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Miosis and Constipation:** These are the two effects of opioids to which **tolerance never develops** [1]. 2. **Biliary Colic:** Opioids cause contraction of the **Sphincter of Oddi**, increasing biliary pressure (Morphine is generally avoided in biliary colic; Pethidine is traditionally preferred due to less spasm). 3. **Respiratory Depression:** This is the most common cause of death in acute opioid poisoning [1], [2]. 4. **Antidote:** Naloxone is the drug of choice for opioid overdose.

Question 664: A patient on lithium therapy was found to be hypertensive. Which of the following antihypertensive drugs is contraindicated in a patient on lithium therapy in order to prevent toxicity?

A. Clonidine
B. Beta blockers
C. Calcium channel blockers
D. Diuretics (Correct Answer)

Explanation: ### Explanation **Mechanism of Interaction (Why Diuretics are Correct):** Lithium is a monovalent cation that is handled by the kidneys similarly to sodium. It is freely filtered at the glomerulus and approximately 80% is reabsorbed in the proximal convoluted tubule (PCT). **Thiazide and Loop diuretics** cause sodium depletion. When the body senses low sodium, the PCT compensates by increasing the reabsorption of sodium to maintain homeostasis. Because the PCT cannot distinguish between sodium and lithium, it also increases the reabsorption of lithium. This leads to a significant rise in serum lithium levels, potentially resulting in **lithium toxicity** (narrow therapeutic index: 0.6–1.2 mEq/L). **Analysis of Other Options:** * **A. Clonidine:** This centrally acting alpha-2 agonist does not significantly affect renal lithium clearance and is generally safe to use. * **B. Beta-blockers:** These are often used alongside lithium to treat lithium-induced fine tremors (specifically Propranolol). They do not increase lithium levels. * **C. Calcium Channel Blockers (CCBs):** While some CCBs (like Verapamil) may rarely increase the neurotoxicity of lithium, they do not consistently alter renal clearance or serum levels in the same predictable, dangerous manner as diuretics. **High-Yield Clinical Pearls for NEET-PG:** * **Drugs that INCREASE Lithium levels:** **D**iuretics (Thiazides > Loop), **N**SAIDs (except Aspirin/Sulindac), and **A**CE inhibitors/ARBs (Mnemonic: **DNA**). * **Drug that DECREASES Lithium levels:** Acetazolamide, Theophylline, and Caffeine (by increasing renal excretion). * **Side Effects of Lithium:** L-**L**eukocytosis, I-**I**nsipidus (Nephrogenic Diabetes Insipidus), T-**T**remors/Teratogenicity (Ebstein's Anomaly), H-**H**ypothyroidism. * **Drug of choice for Lithium-induced NDI:** Amiloride (blocks lithium entry into collecting duct cells).

Question 665: Pimozide belongs to which class of drugs?

A. Thiothixanthene
B. Phenothiazine
C. Butyrophenone
D. Diphenyl butyl piperidine (Correct Answer)

Explanation: **Explanation:** **Pimozide** is a potent first-generation (typical) antipsychotic. Chemically, it is classified as a **Diphenyl butyl piperidine** derivative. This class is structurally related to butyrophenones but is characterized by a longer duration of action. **Why the correct option is right:** * **Diphenyl butyl piperidine:** Pimozide is the prototype of this class. Its primary mechanism of action involves the potent blockade of postsynaptic **D2 receptors**. Due to its high potency and long half-life, it is frequently used in the management of chronic schizophrenia and is the drug of choice for **Tourette’s syndrome** and **Delusional Parasitosis** (Ekbom syndrome). **Why the other options are incorrect:** * **Thiothixanthene:** This class includes drugs like **Thiothixene** and Flupentixol. They are structurally similar to phenothiazines but contain a carbon atom instead of nitrogen in the central ring. * **Phenothiazine:** This is the largest class of typical antipsychotics, further divided into Aliphatic (Chlorpromazine), Piperidine (Thioridazine), and Piperazine (Fluphenazine) subgroups. * **Butyrophenone:** This class includes **Haloperidol** and Droperidol. While diphenyl butyl piperidines are chemical derivatives of butyrophenones, they are categorized separately in pharmacological classification. **High-Yield Clinical Pearls for NEET-PG:** 1. **Specific Indication:** Pimozide is highly effective for **monosymptomatic hypochondriacal psychosis** (e.g., Delusional Parasitosis). 2. **ECG Changes:** A critical side effect of Pimozide is **QT interval prolongation**, which can lead to Torsades de Pointes; thus, baseline ECG monitoring is recommended. 3. **Potency:** It has a high extrapyramidal side effect (EPS) profile similar to Haloperidol due to its potent D2 antagonism.

Question 666: Which of the following drugs acts by inhibiting the catechol-O-methyltransferase enzyme?

A. Amantadine
B. Selegiline
C. Tolcapone (Correct Answer)
D. Rotigotine

Explanation: ***Tolcapone***- **Tolcapone** is a potent inhibitor of **catechol-O-methyltransferase (COMT)**, an enzyme responsible for metabolizing dopamine and levodopa. - By inhibiting COMT, Tolcapone increases the bioavailability and half-life of **levodopa** in the brain, thereby improving motor symptoms in **Parkinson's disease**.*Amantadine*- Amantadine is thought to enhance the synthesis and release of **dopamine** from presynaptic nerve terminals. - It also acts as a non-competitive antagonist of the **N-methyl-D-aspartate (NMDA) receptor**, providing benefit primarily for **dyskinesia** associated with long-term levodopa use.*Rotigotine*- **Rotigotine** is classified as a non-ergot **dopamine agonist**, meaning it directly stimulates post-synaptic dopamine receptors (D2 and D3). - It is often administered as a **transdermal patch** for continuous dopamine stimulation, helping to manage motor fluctuations in Parkinson's disease.*Selegiline*- **Selegiline** is an inhibitor of **Monoamine Oxidase type B (MAO-B)**, an enzyme subgroup that preferentially metabolizes dopamine in the brain. - By selectively blocking MAO-B, it reduces the breakdown of dopamine, thereby potentiating the effects of endogenous and administered levodopa.

Question 667: A patient took a drug for migraine treatment, following which there was numbness, and his hand appeared as below. What is the possible drug that was taken?

A. Dihydroergotamine (Correct Answer)
B. Sumatriptan
C. Aspirin
D. Butorphanol

Explanation: ***Dihydroergotamine*** - Dihydroergotamine is an **ergot alkaloid** that causes potent, non-selective **vasoconstriction** of both arteries and veins. This can lead to severe peripheral ischemia, a condition known as **ergotism**. - The clinical presentation of numbness, tingling, and cold extremities, as suggested by the patient's symptoms and the appearance of the hand, is a classic manifestation of ergotamine-induced vasospasm. *Sumatriptan* - Sumatriptan is a **triptan**, which is a selective **5-HT1B/1D receptor agonist**. While it does cause vasoconstriction, its effects are more selective for **cranial arteries**. - Though it can cause paresthesias (tingling/numbness), severe peripheral ischemia leading to the signs seen in the image is a much rarer side effect compared to ergot alkaloids. *Butorphanol* - Butorphanol is an **opioid agonist-antagonist** used for pain relief. Its mechanism of action does not involve vasoconstriction. - Common side effects are related to the central nervous system, such as sedation, dizziness, and nausea, not peripheral vascular symptoms. *Aspirin* - Aspirin is a **nonsteroidal anti-inflammatory drug (NSAID)** that inhibits **cyclooxygenase (COX)** and has antiplatelet effects. It does not cause vasoconstriction. - Its primary side effects include **gastrointestinal irritation** and an increased risk of **bleeding**, which are unrelated to the patient's presentation.

Question 668: Which medication can be administered to reduce the frequency of future headache episodes in a 26-year-old female patient who experiences one-sided pulsating headaches accompanied by nausea, vomiting, and sensitivity to light and finds relief in a dimly lit environment?

A. Diazepam
B. Propranolol (Correct Answer)
C. Fluoxetine
D. Alprazolam

Explanation: ***Propranolol***- This is a non-selective **beta-blocker** and is considered a first-line prophylactic agent for the reduction of frequency and severity in patients with **episodic migraine** [1].- The patient's symptoms (unilateral, pulsating headache, **nausea, vomiting, and photophobia**) [2] are classic features of migraine, making **Propranolol** an appropriate choice for maintenance therapy [3].*Alprazolam*- This medication is a **short-acting benzodiazepine** primarily indicated for the acute management of anxiety and panic disorders [3].- It has **no established role** in the long-term prophylactic management of migraine and carries risks of sedation and dependence.*Diazepam*- This is a **long-acting benzodiazepine** commonly used as a muscle relaxant, anxiolytic, and for acute seizure management.- Similar to Alprazolam, it is **not recommended** for headache prophylaxis due to lack of efficacy and significant abuse potential [3].*Fluoxetine*- This drug is a selective serotonin reuptake inhibitor (**SSRI**) primarily used to treat **major depressive disorder** and anxiety [3].- While some antidepressants (like TCAs) are used for migraine prophylaxis, **Fluoxetine** is generally not considered a standard or preferred first-line agent for preventing migraine attacks.

Question 669: Which anti-epileptic drug is commonly associated with gum hypertrophy and dizziness as side effects?

A. Carbamazepine
B. Topiramate
C. Phenytoin (Correct Answer)
D. Levetiracetam

Explanation: ***Phenytoin*** - It is classically associated with **gingival hyperplasia** (gum hypertrophy) due to increased stimulation of **fibroblast activity** and **collagen synthesis** in the gingiva, which is a major distinction from other AEDs. - Dose-dependent **dizziness** and **ataxia** are very common CNS side effects, reflecting its non-linear kinetics and narrow therapeutic index. *Carbamazepine* - Its most characteristic serious side effects include **aplastic anemia** and agranulocytosis, requiring baseline and periodic complete blood counts (CBCs). - It is a potent inducer of **CYP450 enzymes** and frequently causes **hyponatremia** (via Syndrome of Inappropriate Antidiuretic Hormone secretion, SIADH). *Levetiracetam* - This drug is generally well-tolerated but is notably associated with significant **behavioral side effects** such as irritability, aggression, and mood instability. - It does not cause **gum hypertrophy** or significant liver enzyme induction, unlike Phenytoin or Carbamazepine. *Topiramate* - Commonly causes side effects related to cognition, often called **"Dopamax"**, leading to cognitive slowing, difficulty concentrating, and language problems. - Other unique side effects include **weight loss** and the formation of **kidney stones** (nephrolithiasis) due to inhibition of carbonic anhydrase.

Question 670: What is the mechanism of action of Ethosuximide?

A. Inhibits calcium channels in thalamic neurons (Correct Answer)
B. Enhances GABAergic inhibition
C. Blocks sodium channels
D. Increases serotonin levels

Explanation: ***Inhibits calcium channels in thalamic neurons***- Ethosuximide selectively blocks **T-type calcium channels** (**transient type**) primarily found on **thalamic neurons**.- This blockade reduces the **low-threshold burst firing** responsible for the characteristic **3-Hz spike-and-wave discharge** seen in absence seizures.*Enhances GABAergic inhibition*- This mechanism is characteristic of **Benzodiazepines** and **Barbiturates**, which increase the frequency or duration of GABA-A channel opening.- While some broad-spectrum AEDs like **Valproate** also increase GABA levels, this is not the primary mechanism of action for Ethosuximide.*Blocks sodium channels*- Drugs that block neuronal sodium channels, such as **Phenytoin** and **Carbamazepine**, are effective against tonic-clonic and partial seizures.- This mechanism prevents the rapid, sustained firing of action potentials but is largely ineffective against the burst firing characteristic of absence seizures.*Increases serotonin levels*- This is the primary mechanism associated with **Selective Serotonin Reuptake Inhibitors (SSRIs)** and other antidepressants.- It is not relevant to the function of Ethosuximide, which is specific to modulating neuronal excitability through calcium channels.

Practice by Chapter

General Anesthetics

Practice Questions

Local Anesthetics

Practice Questions

Sedative-Hypnotics

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Antiepileptic Drugs

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Antiparkinsonian Drugs

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Opioid Analgesics

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Drugs of Abuse and Addiction

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Psychostimulants

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Hallucinogens

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CNS Stimulants and Cognitive Enhancers

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