Central Nervous System Pharmacology Practice Questions

Q: Which of the following analgesics should not be given in acute myocardial infarction?

Pentazocine. **Explanation:** The primary goal of analgesia in Acute Myocardial Infarction (AMI) is to relieve pain and reduce myocardial oxygen demand. **Why Pentazocine is contraindicated:** Pentazocine is an opioid agonist-antagonist (κ-agonist and μ-antagonist/partial agonist). It possesses **sympathomimetic properties**, which lead to an increase in plasma catecholamines. This results in an **increase in heart rate (tachycardia) and systemic blood pressure**, which significantly increases **myocardial oxygen demand**. In the setting of an AMI, where the heart is already ischemic, this can worsen the infarct size and increase the risk of arrhythmias. Furthermore, it can increase pulmonary artery pressure, increasing the workload on the right ventricle. **Analysis of other options:** * **Morphine:** This is the **drug of choice** for pain relief in AMI. It provides potent analgesia, reduces anxiety (anxiolytic), and has beneficial hemodynamic effects, such as venodilation (reducing preload) and modest arterial dilation (reducing afterload), thereby decreasing myocardial oxygen demand. * **Methadone:** A long-acting μ-opioid agonist. While not the first line for AMI due to its long half-life and risk of QTc prolongation, it does not possess the sympathomimetic "pressor" effects seen with pentazocine. * **Buprenorphine:** A partial μ-agonist. While it can be used for severe pain, it is generally avoided in AMI because its effects are difficult to reverse with Naloxone, but it does not carry the same high risk of increasing cardiac workload as pentazocine. **High-Yield Clinical Pearls for NEET-PG:** * **MONA** mnemonic for AMI: **M**orphine, **O**xygen, **N**itroglycerin, **A**spirin. * **Pentazocine** should also be avoided in patients with heart failure for the same hemodynamic reasons. * **Naloxone** is the specific antagonist for opioid overdose, but higher doses may be required to reverse buprenorphine.

Q: All the following statements regarding remifentanil are true EXCEPT?

Long acting anesthetic. Remifentanil is a unique, ultra-short-acting synthetic opioid analgesic used primarily in anesthesia. The correct answer is **D (Long acting anesthetic)** because remifentanil is characterized by its exceptionally short duration of action, not a long one [1]. **1. Why Option D is the correct answer (The Exception):** Remifentanil has a unique chemical structure containing an ester linkage. This allows it to undergo rapid hydrolysis by **non-specific tissue and plasma esterases**. Consequently, it has an ultra-short half-life (approximately 3–10 minutes) and a context-sensitive half-time that remains constant regardless of the duration of infusion [1]. This makes it the shortest-acting opioid available. **2. Analysis of Incorrect Options:** * **Option A (Useful for short painful procedures):** Due to its rapid onset and offset, it is ideal for procedures requiring intense analgesia for a brief period (e.g., intubation, brief surgeries) without prolonged respiratory depression. * **Option B (Metabolized by plasma esterases):** This is its defining pharmacokinetic feature. Unlike other opioids metabolized by the liver (CYP450), remifentanil metabolism is independent of hepatic or renal function. * **Option C (Equipotent as fentanyl):** Remifentanil and fentanyl are considered roughly equipotent in terms of analgesic strength, though remifentanil acts much faster and wears off sooner. **Clinical Pearls for NEET-PG:** * **Context-Sensitive Half-Time:** Remifentanil is the only opioid where this remains nearly constant (~3 mins) even after a 10-hour infusion. * **Organ-Independent Elimination:** It is the drug of choice for patients with end-stage liver or renal failure. * **Opioid-Induced Hyperalgesia:** Rapid withdrawal or high doses of remifentanil can sometimes lead to acute postoperative hyperalgesia. * **Mnemonic:** **R**emifentanil = **R**apidly metabolized by **R**BC/Plasma esterases.

Q: Epinephrine added to a solution of lignocaine for a peripheral nerve block will:

Increase the duration of action of the local anesthetic. ### Explanation **1. Why Option B is Correct:** Lignocaine is a local anesthetic (LA) that causes vasodilation at the site of injection. When **Epinephrine (Adrenaline)** is added, it acts on **$\alpha_1$-adrenergic receptors** to cause potent **vasoconstriction**. This results in: * **Decreased systemic absorption:** The LA remains at the nerve site for a longer period. * **Increased duration of action:** Since the drug is not washed away by the bloodstream quickly, the block lasts longer. * **Reduced systemic toxicity:** Slower absorption leads to lower peak plasma levels of the LA. * **Decreased bleeding:** Vasoconstriction provides a bloodless field for minor procedures. **2. Why Other Options are Incorrect:** * **Option A:** Epinephrine actually **decreases** the risk of convulsions. Convulsions are a sign of systemic CNS toxicity (due to high plasma levels of LA). By slowing systemic absorption, epinephrine keeps plasma levels low, thereby reducing the risk of CNS and cardiac toxicity. * **Option C:** Incorrect because the risk of convulsions is decreased, not increased. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Standard Concentration:** Epinephrine is typically added to LAs in a concentration of **1:200,000**. * **Contraindications:** Never use LA with Epinephrine for blocks in **"end-artery" areas** (fingers, toes, nose, ear lobules, and penis) as it can cause ischemia and **gangrene**. * **Hypertension/Cardiac Disease:** Use with caution in patients with uncontrolled hypertension or ischemic heart disease due to potential systemic effects of adrenaline. * **Felypressin:** An alternative vasoconstrictor (vasopressin analogue) sometimes used in dental anesthesia when epinephrine is contraindicated.

Q: Which of the following local anesthetics has the longest duration of action?

Tetracaine. **Explanation:** The duration of action of a local anesthetic (LA) is primarily determined by its **protein binding capacity**. Agents with high affinity for plasma and tissue proteins remain at the nerve receptor site for a longer period. **Why Tetracaine is Correct:** Tetracaine is an ester-linked local anesthetic known for its high lipid solubility and high protein binding. Among the options provided, **Tetracaine** has the longest duration of action (approximately 2–3 hours, which can be extended to 4–6 hours with epinephrine). While Bupivacaine is also long-acting, in standard pharmacological classifications and comparative potency charts used in exams, Tetracaine (especially in spinal anesthesia) is categorized as having a longer duration than Bupivacaine. **Analysis of Incorrect Options:** * **Bupivacaine (Option A):** An amide-linked LA with a long duration of action (approx. 120–240 minutes). While very common clinically, it is slightly shorter-acting than Tetracaine in specific formulations. * **Lidocaine (Option C):** An amide-linked LA with **intermediate** duration (30–60 minutes). It is the most commonly used LA for infiltration and regional blocks. * **Procaine (Option D):** An ester-linked LA with a **short** duration of action (15–30 minutes) due to rapid hydrolysis by plasma pseudocholinesterase. **High-Yield Clinical Pearls for NEET-PG:** * **Potency** is determined by **Lipid Solubility**. * **Duration of Action** is determined by **Protein Binding**. * **Onset of Action** is determined by the **pKa** (lower pKa = faster onset, as more drug exists in the uncharged base form). * **Metabolism:** Esters (like Tetracaine/Procaine) are metabolized by **plasma pseudocholinesterase**; Amides (like Lidocaine/Bupivacaine) are metabolized by **liver microsomal enzymes**. * **Bupivacaine** is notorious for **cardiotoxicity** (blocks cardiac sodium channels during diastole).

Q: What is the primary mechanism of action of local anesthetics?

Block the influx of sodium into the cell. **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.

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

Need to cross the cell membrane to produce the block. **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.

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

Anti-emesis. **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.

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

Tolcapone. ***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 1

Which of the following analgesics should not be given in acute myocardial infarction?

Accepted Answer

Pentazocine

Answer

Methadone

Answer

Morphine

Answer

Buprenorphine

Question 2

All the following statements regarding remifentanil are true EXCEPT?

Accepted Answer

Long acting anesthetic

Answer

Useful for short painful procedures

Answer

Metabolized by plasma esterases

Answer

Equipotent as fentanyl

Question 3

Epinephrine added to a solution of lignocaine for a peripheral nerve block will:

Accepted Answer

Increase the duration of action of the local anesthetic

Answer

Increase risk of convulsions

Answer

Both increase risk of convulsions and increase the duration of action of the local anesthetic

Answer

None of these

Question 4

Which of the following local anesthetics has the longest duration of action?

Accepted Answer

Tetracaine

Answer

Bupivacaine

Answer

Lidocaine

Answer

Procaine

Question 5

Naltrexone is used to maintain abstinence following opioid withdrawal in addicts. It blocks which of the following features of opioid use, except?

Accepted Answer

Craving for opioids

Answer

Euphoriant effects of opioids

Answer

Miosis

Answer

Respiratory depression

Question 6

Which antiepileptic drug is the DOC of Generalized Tonic-Clonic Seizures (GTCS) in pregnancy?

Accepted Answer

Lamotrigine

Answer

Carbamazepine

Answer

Levetiracetam

Answer

Valproate

Question 7

What is the primary mechanism of action of local anesthetics?

Accepted Answer

Block the influx of sodium into the cell

Answer

Block the release of neurotransmitters

Answer

Increase the release of inhibitory neurotransmitters

Answer

Inhibit the efflux of sodium from neurons

Question 8

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

Accepted Answer

Need to cross the cell membrane to produce the block

Answer

Greater potential to block a resting nerve as compared to a stimulated nerve

Answer

Large myelinated fibers are blocked before small myelinated fibers

Answer

Causes a consistent change in resting membrane potential

Question 9

Which of the following is NOT a pharmacological action of opioids?

Accepted Answer

Anti-emesis

Answer

Cough suppression

Answer

Miosis

Answer

Truncal rigidity

Question 10

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

Accepted Answer

Tolcapone

Answer

Amantadine

Answer

Selegiline

Answer

Rotigotine

Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology — MCQs

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