Central Nervous System Pharmacology Practice Questions

Q: Which of the following best describes the mechanism of action of memantine when used for Alzheimer's disease?

NMDA receptor antagonist. ***NMDA receptor antagonist*** - Memantine acts as an uncompetitive **NMDA (N-methyl-D-aspartate) receptor antagonist**, which helps to reduce the excitotoxic effects of **glutamate** in the brain. - By modulating glutamate activity, it prevents the chronic overstimulation of these receptors, which is associated with **neurodegeneration** in Alzheimer's disease. *Acetylcholinesterase inhibition* - This is the mechanism of action for drugs like **donepezil**, **rivastigmine**, and **galantamine**. - These drugs increase the availability of **acetylcholine** in the synaptic cleft to improve cognitive function, rather than acting on NMDA receptors. *Blockade of A-beta amyloid* - Monoclonal antibodies such as **aducanumab** or **lecanemab** are designed to target and clear **amyloid-beta plaques** in the brain. - This approach focuses on disease modification (clearing pathological proteins) rather than the symptomatic modulation of neuronal excitability provided by memantine. *Dopamine D2 receptor antagonism* - This is the mechanism characteristic of most **antipsychotic medications** (e.g., risperidone, haloperidol). - Blocking D2 receptors is primarily used to manage psychiatric symptoms like hallucinations or agitation, not for the primary cognitive symptoms of Alzheimer's disease.

Q: Lamotrigine is most effective in which of the following seizure types?

Tonic-clonic seizures. ***Tonic-clonic seizures*** - Lamotrigine is a **broad-spectrum** anticonvulsant that is highly effective for the management of **generalized tonic-clonic seizures**. - It works by blocking **voltage-gated sodium channels** and inhibiting the release of excitatory neurotransmitters like **glutamate**. *Absence seizures* - **Ethosuximide** is the first-line treatment for uncomplicated absence seizures, not lamotrigine. - While lamotrigine has some activity against absence seizures, it is not the primary drug of choice for this specific type. *Atonic seizures* - Atonic seizures (drop attacks) are typically refractory and often treated with **valproate** or as part of Lennox-Gastaut syndrome management. - While lamotrigine is used in generalized epilepsy syndromes, it is not the defining or most specific indication compared to tonic-clonic seizures. *Myoclonic seizures* - **Valproate** is widely considered the most effective agent for myoclonic seizures, particularly in the context of juvenile myoclonic epilepsy. - Lamotrigine is often used as an adjunct or alternative, but it is not the classic first-line choice for pure myoclonic epilepsy.

Q: Erenumab was approved by FDA in 2018 for which condition?

Migraine. **Explanation:** **Erenumab** is a breakthrough therapeutic agent in the management of **Migraine**. It is a fully human monoclonal antibody that specifically targets and blocks the **Calcitonin Gene-Related Peptide (CGRP) receptor**. CGRP is a potent neuropeptide and vasodilator that plays a central role in the pathophysiology of migraine by mediating pain transmission and neurogenic inflammation in the trigeminal system. By antagonizing this receptor, Erenumab effectively prevents the initiation of migraine attacks. It was the first FDA-approved drug (2018) in this class specifically for the **prophylaxis** of migraine in adults. **Analysis of Incorrect Options:** * **A & D (Phenylketonuria & Glycogen storage disorders):** These are metabolic/genetic disorders. Phenylketonuria is typically managed with dietary restriction or drugs like Sapropterin. Glycogen storage diseases are managed through nutritional support or enzyme replacement therapy (e.g., Alglucosidase alfa for Pompe disease), not monoclonal antibodies targeting neuropeptides. * **C (Hypertension):** While CGRP is a vasodilator, Erenumab is not used for hypertension. In fact, because it blocks a vasodilator, a potential side effect of Erenumab is a slight increase in blood pressure. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** CGRP Receptor Antagonist (Contrast this with *Fremanezumab* and *Galcanezumab*, which bind to the CGRP **ligand** itself). * **Route:** Administered via subcutaneous injection once monthly. * **Indications:** Prophylaxis of both episodic and chronic migraine. * **Side Effects:** Most common are injection site reactions and constipation. * **Other "Gepants":** Oral CGRP antagonists (e.g., Rimegepant, Ubrogepant) are used for acute treatment or prevention.

Q: All except one are centrally acting muscle relaxants. Which one is not?

Dantrolene sodium. **Explanation:** Skeletal muscle relaxants are broadly classified into two categories based on their site of action: **Centrally acting** (acting on the CNS) and **Peripherally acting** (acting at the neuromuscular junction or directly on the muscle fiber). **1. Why Dantrolene Sodium is the correct answer:** Dantrolene is a **peripherally acting** muscle relaxant. Unlike the other options, it does not work by depressing the central nervous system. Instead, it acts directly on the skeletal muscle by binding to the **Ryanodine Receptor (RyR1)** on the sarcoplasmic reticulum. This inhibits the release of calcium ions into the cytosol, thereby preventing muscle contraction (excitation-contraction uncoupling). **2. Analysis of Incorrect Options:** * **Meprobamate:** A carbamate derivative primarily used as an anxiolytic; it acts as a central depressant with muscle relaxant properties. * **Baclofen:** A **GABA-B agonist** that acts at the spinal cord level to reduce spasticity by inhibiting monosynaptic and polysynaptic reflexes. * **Diazepam:** A Benzodiazepine that acts via **GABA-A receptors** in the CNS to increase chloride conductance, leading to hyperpolarization and muscle relaxation. **Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Dantrolene is the DOC for **Malignant Hyperthermia** (caused by halothane/succinylcholine) and **Neuroleptic Malignant Syndrome (NMS)**. * **Baclofen** is the DOC for spasticity in Multiple Sclerosis but is ineffective in stroke. * **Tizanidine** is another high-yield centrally acting relaxant that works as a central **$\alpha_2$ agonist**. * **Side Effect:** A major side effect of Dantrolene is hepatotoxicity (monitor LFTs).

Q: Which of the following acts as an antagonist to the NMDA receptor?

Ketamine. **Explanation:** The **NMDA (N-methyl-D-aspartate) receptor** is an ionotropic glutamate receptor that plays a critical role in synaptic plasticity and memory. It is unique because it requires both glutamate binding and glycine as a co-agonist, along with the removal of a magnesium (Mg²⁺) plug via depolarization. **Ketamine** (Option A) is a dissociative anesthetic that acts as a **non-competitive antagonist** at the NMDA receptor. It binds to the phencyclidine (PCP) site inside the ion channel, effectively blocking the flow of cations. This blockade results in "dissociative anesthesia," characterized by profound analgesia, amnesia, and a cataleptic state. **Analysis of Incorrect Options:** * **Spermine (Option B):** This is a polyamine that acts as a **positive allosteric modulator** (agonist-like effect) at the NMDA receptor, facilitating its function rather than inhibiting it. * **Muscimol (Option C):** This is a potent, selective **GABA-A receptor agonist** derived from the mushroom *Amanita muscaria*. * **Baclofen (Option D):** This is a selective **GABA-B receptor agonist** used clinically as a centrally acting muscle relaxant to treat spasticity. **High-Yield Clinical Pearls for NEET-PG:** * **Other NMDA Antagonists:** Memantine (used in Alzheimer’s), Amantadine (Parkinson’s), Dextromethorphan (antitussive), and Phencyclidine (PCP). * **Ketamine Side Effects:** It is notorious for causing **emergence delirium** and hallucinations. It is also the anesthetic of choice in patients with **asthma** (due to bronchodilation) and **hypovolemic shock** (due to sympathetic stimulation). * **Contraindication:** Ketamine should be avoided in patients with increased intracranial pressure (ICP).

Q: According to signal transduction mechanisms, what type of receptor is the GABA-B receptor?

G protein-coupled receptor. ### Explanation The **GABA-B receptor** is a metabotropic receptor, meaning it exerts its effects through an intermediary signaling molecule. Specifically, it is a **G protein-coupled receptor (GPCR)** coupled to the **Gᵢ/Gₒ protein** subtype. **Mechanism of Action:** Activation of GABA-B receptors leads to: 1. **Inhibition of Adenylyl Cyclase:** Decreasing intracellular cAMP levels. 2. **Opening of K⁺ channels:** Causing potassium efflux and membrane hyperpolarization (postsynaptic effect). 3. **Closing of Ca²⁺ channels:** Inhibiting neurotransmitter release (presynaptic effect). --- ### Analysis of Incorrect Options: * **A. Ligand-gated ion channel receptor:** This describes the **GABA-A receptor**, which is an ionotropic receptor. Upon activation, it directly opens a chloride (Cl⁻) channel, leading to rapid hyperpolarization. * **B. Nuclear receptor:** These are intracellular receptors for lipid-soluble ligands (e.g., steroids, thyroid hormones) that act as transcription factors. GABA is a neurotransmitter that acts on cell-surface receptors. * **C. Enzyme-linked receptor:** These receptors (e.g., Insulin, Growth Factors) have intrinsic enzymatic activity (like Tyrosine Kinase). GABA-B signaling does not involve direct enzymatic activation. --- ### High-Yield Clinical Pearls for NEET-PG: * **Baclofen:** A selective **GABA-B agonist** used clinically as a centrally acting muscle relaxant to treat spasticity (e.g., in Multiple Sclerosis or spinal cord injuries). * **Location:** GABA-A is primarily postsynaptic, whereas GABA-B is found both **presynaptically** (acting as an autoreceptor) and postsynaptically. * **Response Time:** GABA-A mediates "fast" inhibitory postsynaptic potentials (IPSPs), while GABA-B mediates "slow" and prolonged IPSPs due to the second-messenger cascade.

Q: What is the preferred drug for alcohol withdrawal seizures?

Diazepam. **Explanation:** **1. Why Diazepam is the Correct Answer:** Alcohol withdrawal leads to a state of **CNS hyperexcitability** due to the sudden removal of GABAergic inhibition and compensatory upregulation of NMDA (glutamate) receptors. **Benzodiazepines (BZDs)**, such as **Diazepam**, are the gold standard for managing alcohol withdrawal symptoms and seizures. They act as positive allosteric modators of the $GABA_A$ receptor, providing **cross-tolerance** with alcohol. This restores the inhibitory tone, effectively suppressing withdrawal seizures and preventing progression to Delirium Tremens. Diazepam is preferred due to its rapid onset and long-acting metabolites, which provide a "self-tapering" effect. **2. Why Other Options are Incorrect:** * **Valproate & Carbamazepine:** While these are potent anti-epileptics, they do not address the underlying GABA-NMDA imbalance of alcohol withdrawal as effectively as BZDs. They are generally considered second-line or adjunct treatments and do not prevent Delirium Tremens. * **Phenobarbitone:** This is a barbiturate that also acts on GABA receptors. While effective in refractory cases or ICU settings, it has a narrower therapeutic index and a higher risk of respiratory depression compared to BZDs, making it a second-line choice. **3. Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** For alcohol withdrawal seizures/delirium is **Diazepam** or **Chlordiazepoxide**. * **Liver Impairment Exception:** If the patient has significant liver cirrhosis, use **LOT** (Lorazepam, Oxazepam, Temazepam) because they undergo direct glucuronidation and do not have active metabolites that burden the liver. * **Prophylaxis:** BZDs are the only class proven to reduce the incidence of seizures and delirium in withdrawal. * **Phenytoin:** Notably **ineffective** for alcohol withdrawal seizures and should not be used.

Q: Which of the following statements regarding local anesthesia are true?

It inhibits the generation of action potential.. **Explanation:** **Mechanism of Action (Why A is correct):** Local anesthetics (LAs) work by blocking **voltage-gated sodium channels** from the intracellular side of the neuronal membrane. By binding to these channels, they prevent the influx of sodium ions required for depolarization. Consequently, the threshold for excitation is not reached, and the **generation and conduction of action potentials** are inhibited. **Analysis of Incorrect Options:** * **Option B:** While thin fibers are generally blocked more easily than thick fibers, **myelination** actually increases susceptibility. Myelinated fibers are blocked more rapidly than unmyelinated fibers of the same diameter because LAs concentrate their action at the **Nodes of Ranvier**. * **Option C:** Vasoconstrictors (like Adrenaline) reduce *systemic absorption* and prolong the duration of action, but they do not inherently reduce the intrinsic toxicity of the drug. In fact, if accidentally injected intravascularly, the addition of adrenaline can worsen cardiovascular side effects (tachycardia/hypertension). * **Option D:** LAs do not block all modalities simultaneously. There is a characteristic **differential nerve block**. The typical order of disappearance is: **Pain > Temperature > Touch > Deep Pressure > Motor function.** **NEET-PG High-Yield Pearls:** * **State-Dependent Block:** LAs have a higher affinity for sodium channels in the **activated (open)** or **inactivated** states rather than the resting state. * **pH Sensitivity:** LAs are weak bases. In acidic environments (e.g., infected tissue/abscess), they become ionized and cannot cross the lipid membrane, leading to **reduced efficacy**. * **Bupivacaine:** Most cardiotoxic LA; **Intralipid (20% lipid emulsion)** is the specific antidote for Local Anesthetic Systemic Toxicity (LAST).

Q: Which of the following statements is false regarding phenobarbitone?

Has no significant interaction with warfarin. **Explanation:** The correct answer is **D** because the statement "Has no significant interaction with warfarin" is **false**. Phenobarbitone is a potent **microsomal enzyme inducer** (specifically inducing CYP2C9 and CYP3A4). It accelerates the metabolism of warfarin, thereby decreasing its plasma concentration and anticoagulant efficacy. Patients on this combination require a higher dose of warfarin to maintain a therapeutic INR. **Analysis of other options:** * **Option A (True):** Phenobarbitone acts on the GABA$_A$ receptor-chloride channel complex. Unlike benzodiazepines, it increases the **duration** of chloride channel opening and can also act as a GABA-mimetic at high doses. * **Option B (True):** Phenobarbitone is lipid-soluble and easily **crosses the placenta**. It can cause neonatal respiratory depression and "fetal hydantoin-like syndrome" (though less common than with phenytoin) or neonatal hemorrhage due to Vitamin K deficiency. * **Option C (True):** Barbiturates induce the enzyme **ALA synthetase**, which increases porphyrin synthesis. This can precipitate acute intermittent porphyria in susceptible individuals; thus, it is strictly contraindicated. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Phenobarbitone remains the drug of choice for **Neonatal Seizures**. * **Metabolism:** It follows first-order kinetics (unlike phenytoin which follows zero-order). * **Alkalinization of Urine:** Since it is a weak acid, its excretion is enhanced by sodium bicarbonate in cases of toxicity. * **Enzyme Induction:** It also induces the metabolism of oral contraceptives (leading to failure), steroids, and vitamin D (leading to osteomalacia).

Question 1

Which of the following best describes the mechanism of action of memantine when used for Alzheimer's disease?

Accepted Answer

NMDA receptor antagonist

Answer

Acetylcholinesterase inhibition

Answer

Blockade of A-beta amyloid

Answer

Dopamine D2 receptor antagonism

Question 2

Which of the following best describes the mechanism of action of gabapentin?

Accepted Answer

Blockage of the α2δ subunit of voltage -gated calcium channels

Answer

Sodium channel blockade

Answer

Opioid receptor blockade

Answer

NMDA receptor antagonism

Question 3

Lamotrigine is most effective in which of the following seizure types?

Accepted Answer

Tonic-clonic seizures

Answer

Absence seizures

Answer

Atonic seizures

Answer

Myoclonic seizures

Question 4

Erenumab was approved by FDA in 2018 for which condition?

Accepted Answer

Migraine

Answer

Phenylketonuria

Answer

Hypertension

Answer

Glycogen storage disorders

Question 5

All except one are centrally acting muscle relaxants. Which one is not?

Accepted Answer

Dantrolene sodium

Answer

Meprobamate

Answer

Baclofen

Answer

Diazepam

Question 6

Which of the following acts as an antagonist to the NMDA receptor?

Accepted Answer

Ketamine

Answer

Spermine

Answer

Muscimol

Answer

Baclofen

Question 7

According to signal transduction mechanisms, what type of receptor is the GABA-B receptor?

Accepted Answer

G protein-coupled receptor

Answer

Ligand-gated ion channel receptor

Answer

Nuclear receptor

Answer

Enzyme-linked receptor

Question 8

What is the preferred drug for alcohol withdrawal seizures?

Accepted Answer

Diazepam

Answer

Valproate

Answer

Phenobarbitone

Answer

Carbamazepine

Question 9

Which of the following statements regarding local anesthesia are true?

Accepted Answer

It inhibits the generation of action potential.

Answer

Unmyelinated thin fibers are more susceptible than myelinated large fibers.

Answer

Toxicity is reduced by the addition of vasoconstrictors.

Answer

It blocks all modalities of sensation at the same time.

Question 10

Which of the following statements is false regarding phenobarbitone?

Accepted Answer

Has no significant interaction with warfarin

Answer

Acts primarily at the GABA-BZD receptor-chloride ion channel complex

Answer

Crosses the placenta

Answer

May precipitate porphyria

Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology — MCQs

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