Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 161: What is the first-line drug for absence seizures?

A. Phenytoin
B. Benzodiazepines
C. Valproate (Correct Answer)
D. Carbamazepine

Explanation: **Explanation:** **1. Why Valproate is the Correct Answer:** Valproate (Sodium Valproate) is considered the first-line treatment for absence seizures, especially when they coexist with other seizure types like Generalized Tonic-Clonic Seizures (GTCS). Its mechanism involves multiple pathways: it inhibits T-type calcium channels in thalamic neurons (the hallmark of absence seizures), increases GABA levels, and blocks voltage-gated sodium channels. While **Ethosuximide** is also a first-line choice specifically for pure absence seizures, Valproate is often preferred in clinical practice and exams due to its broad-spectrum activity against multiple seizure types. **2. Why the Other Options are Incorrect:** * **Phenytoin (A) and Carbamazepine (D):** These are narrow-spectrum anticonvulsants that act primarily by blocking sodium channels. Crucially, they are **contraindicated** in absence seizures as they can paradoxically aggravate or precipitate absence attacks and myoclonic seizures. * **Benzodiazepines (B):** While drugs like Clonazepam are effective against absence seizures, they are generally reserved as second or third-line agents due to the development of tolerance and side effects like sedation. **3. Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC) Summary:** * Pure Absence Seizures: Ethosuximide (preferred in children to avoid hepatotoxicity). * Absence + GTCS: Valproate. * Myoclonic Seizures: Valproate. * **EEG Hallmark:** Absence seizures are characterized by a classic **3 Hz spike-and-wave pattern**. * **Teratogenicity:** Valproate is highly teratogenic (causes Neural Tube Defects); avoid in women of childbearing age if possible.

Question 162: Local anaesthetics act by inhibiting which of the following?

A. Influx of K+
B. Efflux of K+
C. Influx of Na+ (Correct Answer)
D. Efflux of Na+

Explanation: **Explanation:** Local anesthetics (LAs) exert their primary effect by blocking **voltage-gated sodium channels** on the neuronal cell membrane. **Why Influx of Na+ is the Correct Answer:** The generation and conduction of an action potential depend on the rapid entry of sodium ions into the cell. LAs are weak bases that exist in an equilibrium between uncharged and charged forms. The uncharged form diffuses across the axonal membrane and re-ionizes inside the cell. This ionized form then binds to a specific receptor site within the inner pore of the sodium channel. By plugging the channel, LAs prevent the **influx of Na+**, thereby inhibiting depolarization. If the threshold potential cannot be reached, the action potential fails to propagate, leading to a loss of sensation (anesthesia). **Analysis of Incorrect Options:** * **Efflux of K+:** Potassium efflux is responsible for the repolarization phase of the action potential. While some LAs can affect K+ channels at very high concentrations, this is not their primary mechanism for clinical anesthesia. * **Influx of K+:** Under physiological conditions, K+ does not typically flow into the cell during an action potential (it moves out down its concentration gradient). * **Efflux of Na+:** Sodium efflux occurs via the Na+/K+ ATPase pump to restore resting membrane potential; it is not the mechanism inhibited by LAs to block nerve conduction. **High-Yield NEET-PG Pearls:** 1. **State-Dependent Block:** LAs have a higher affinity for channels in the **Activated (Open)** and **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 (Type B and Aδ) are blocked before large, unmyelinated fibers. Clinically, the sequence is: **Pain > Temperature > Touch > Deep Pressure > Motor.** 3. **pH Effect:** LAs are less effective in **inflamed/acidic tissues** because the drug becomes ionized outside the cell and cannot cross the lipid membrane.

Question 163: What is the drug of choice for treating phenothiazine-induced parkinsonism?

A. Levodopa
B. Benserazide
C. Benzhexol (Correct Answer)
D. Selegiline

Explanation: **Explanation:** The correct answer is **Benzhexol** (also known as Trihexyphenidyl). **Mechanism and Rationale:** Phenothiazines (antipsychotics) cause parkinsonism by blocking dopamine ($D_2$) receptors in the nigrostriatal pathway. This creates a functional imbalance in the basal ganglia: **decreased dopaminergic activity** leads to a relative **excess of cholinergic activity**. To correct this imbalance in drug-induced parkinsonism (DIP), we use **centrally acting anticholinergics** like Benzhexol, Benztropine, or Biperiden. These drugs block muscarinic receptors, restoring the dopamine-acetylcholine balance without interfering with the antipsychotic effect of the phenothiazine. **Why other options are incorrect:** * **Levodopa:** It is the gold standard for idiopathic Parkinson’s disease but is **contraindicated** in DIP. Increasing dopamine levels can exacerbate the underlying psychosis for which the phenothiazine was prescribed. Furthermore, since the receptors are already blocked by the phenothiazine, Levodopa is largely ineffective. * **Benserazide:** This is a peripheral dopa-decarboxylase inhibitor used only in combination with Levodopa to prevent its peripheral metabolism. It has no therapeutic effect on its own. * **Selegiline:** An MAO-B inhibitor used as an adjunct in idiopathic Parkinson’s. It is not used for DIP because it increases dopamine levels, risking a relapse of psychotic symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice for DIP:** Central anticholinergics (Benzhexol). * **Drug of Choice for Akathisia:** Propranolol. * **Drug of Choice for Acute Dystonia:** Intravenous/Intramuscular Promethazine or Benztropine. * **Tardive Dyskinesia:** Anticholinergics actually **worsen** this condition; it is managed by stopping the offending drug or switching to Clozapine.

Question 164: All of the following agents are used for prophylaxis of migraine, except?

A. Propranolol
B. Valproate
C. Topiramate
D. Ethosuximide (Correct Answer)

Explanation: **Explanation:** The correct answer is **Ethosuximide** because it has no clinical role in migraine prophylaxis. Its mechanism of action involves blocking **T-type calcium channels** in the thalamus, making it the drug of choice specifically for **Absence Seizures (Petit Mal)**, but it does not affect the neurovascular pathways involved in migraine. **Analysis of Options:** * **Propranolol (Option A):** A non-selective beta-blocker and the **first-line agent** for migraine prophylaxis. It works by stabilizing vascular tone and reducing central sympathetic excitability. * **Valproate (Option B):** An antiepileptic that increases GABA levels and modulates glutamate. It is highly effective for preventing migraine attacks, though contraindicated in pregnancy due to teratogenicity (neural tube defects). * **Topiramate (Option C):** An antiepileptic that blocks sodium channels and antagonizes glutamate receptors. It is a first-line prophylactic agent, especially useful in patients who are overweight, as it causes weight loss. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis Criteria:** Indicated if attacks occur >2–3 times/month or are severe/disabling. * **First-line Prophylactic Agents:** Beta-blockers (Propranolol, Timolol), Anticonvulsants (Topiramate, Valproate), and Calcium Channel Blockers (Flunarizine). * **Newer Agents:** CGRP antagonists (e.g., Erenumab) are used for refractory cases. * **Acute Treatment:** Triptans (5-HT 1B/1D agonists) are the gold standard for aborting an acute attack, but they are **not** used for prophylaxis. * **Amitriptyline:** A TCA often used for prophylaxis when the patient also has comorbid depression or insomnia.

Question 165: Which of the following is NOT a side effect of phenytoin?

A. Osteomalacia
B. Maculopapular rash
C. Sedation (Correct Answer)
D. Megaloblastic anaemia

Explanation: Phenytoin is a first-line antiepileptic drug used for focal and generalized tonic-clonic seizures. Unlike many other CNS depressants (such as barbiturates or benzodiazepines), **phenytoin does not cause significant sedation** at therapeutic doses. This is a key clinical advantage, as it allows for seizure control without impairing the patient’s level of consciousness. **Explanation of Options:** * **Sedation (Correct Answer):** Phenytoin acts by blocking voltage-gated sodium channels in their inactivated state. This selective action prevents high-frequency repetitive firing without causing generalized CNS depression or sedation. * **Osteomalacia:** Phenytoin induces hepatic cytochrome P450 enzymes, which increases the metabolism of Vitamin D. This leads to Vitamin D deficiency, hypocalcemia, and subsequent osteomalacia (in adults) or rickets (in children). * **Maculopapular rash:** Hypersensitivity reactions are common with phenytoin, ranging from mild morbilliform rashes to life-threatening conditions like Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN), especially in patients with the HLA-B*1502 allele. * **Megaloblastic Anaemia:** Phenytoin interferes with the absorption and metabolism of dietary folate. Chronic use can lead to folate deficiency, manifesting as macrocytic/megaloblastic anaemia. **NEET-PG High-Yield Pearls (Mnemonic: HOT MALAI):** * **H**irsutism, **H**ypertrophy of gums (Gingival hyperplasia due to increased PDGF). * **O**steomalacia. * **T**eratogenicity (Fetal Hydantoin Syndrome: cleft lip/palate, digital hypoplasia). * **M**egaloblastic anaemia. * **A**rrhythmias (on rapid IV injection). * **L**ymphadenopathy (Pseudolymphoma). * **A**taxia and nystagmus (signs of toxicity). * **I**nsulin inhibition (leading to hyperglycemia). **Note:** Phenytoin follows **Zero-order kinetics** (saturation kinetics) at therapeutic concentrations, meaning small dose increases can lead to disproportionately large increases in plasma levels and toxicity.

Question 166: Which of the following is not a feature of fetal alcohol syndrome?

A. Microcephaly
B. Low intelligence
C. Large proportionate body (Correct Answer)
D. Intrauterine growth retardation

Explanation: **Explanation:** Fetal Alcohol Syndrome (FAS) occurs due to the teratogenic effects of maternal alcohol consumption during pregnancy. Alcohol crosses the placental barrier, leading to multi-organ developmental defects [1], [2]. **Why "Large proportionate body" is the correct answer:** Alcohol is a potent inhibitor of cell proliferation and protein synthesis. Instead of large size, FAS is characterized by **Intrauterine Growth Retardation (IUGR)** and postnatal growth deficiency [1]. Affected infants are typically small for gestational age and remain in the lower percentiles for height and weight throughout childhood. Therefore, a "large proportionate body" is the opposite of what is clinically observed. **Analysis of incorrect options:** * **Microcephaly (A):** Alcohol is neurotoxic to the developing brain. Reduced brain volume and head circumference (microcephaly) are hallmark features [2]. * **Low intelligence (B):** Alcohol is the leading preventable cause of intellectual disability worldwide. It causes structural brain abnormalities leading to cognitive impairment and behavioral issues [1]. * **Intrauterine growth retardation (D):** As mentioned, alcohol restricts fetal growth, leading to permanent deficits in size and weight [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Facial Features:** Look for the classic triad: **Short palpebral fissures**, **Smooth philtrum**, and a **Thin upper lip (vermilion border)** [1]. * **Cardiac Defects:** The most common association is a **Ventricular Septal Defect (VSD)**. * **Mechanism:** Alcohol induces oxidative stress and interferes with cell adhesion molecules (like L1-CAM), disrupting neuronal migration [2]. * **Safe Limit:** There is **no known safe amount** of alcohol consumption during pregnancy.

Question 167: Which of the following drugs is used as an antismoking agent?

A. Trimethoprim
B. Fluvoxamine
C. Biperiden
D. Mecamylamine (Correct Answer)

Explanation: **Explanation:** **Mecamylamine** is the correct answer because it is a **ganglionic blocker** that acts as a non-competitive antagonist at nicotinic acetylcholine receptors (nAChRs). In the context of smoking cessation, it crosses the blood-brain barrier and blocks the rewarding effects of nicotine in the central nervous system. By inhibiting the "rush" or reinforcement associated with smoking, it helps reduce nicotine craving and prevents relapse. It is often studied in combination with nicotine patches to improve quit rates. **Analysis of Incorrect Options:** * **A. Trimethoprim:** This is a dihydrofolate reductase inhibitor used as an **antibiotic**, primarily for urinary tract infections (UTIs) and in combination with sulfamethoxazole (Co-trimoxazole). * **B. Fluvoxamine:** This is a **Selective Serotonin Reuptake Inhibitor (SSRI)** used primarily in the treatment of Obsessive-Compulsive Disorder (OCD) and depression. While Bupropion (an atypical antidepressant) is used for smoking cessation, Fluvoxamine is not. * **C. Biperiden:** This is a **centrally acting anticholinergic** drug used to treat Parkinson’s disease and drug-induced extrapyramidal symptoms (EPS). **High-Yield NEET-PG Pearls:** * **First-line smoking cessation agents:** Nicotine Replacement Therapy (NRT), **Varenicline** (partial agonist at $\alpha_4\beta_2$ nicotinic receptors—the most effective monotherapy), and **Bupropion** (atypical antidepressant). * **Mecamylamine** was originally developed as an antihypertensive, but its use is limited by side effects related to autonomic blockade (e.g., orthostatic hypotension, constipation). * **Cytisine** is another plant-based alkaloid (similar to Varenicline) gaining importance as a smoking cessation aid.

Question 168: Buprenorphine is:

A. Partial agonist at mu receptor (Correct Answer)
B. Partial agonist at kappa receptor
C. Full agonist at mu receptor
D. Full agonist at kappa receptor

Explanation: **Explanation:** Buprenorphine is a semi-synthetic highly lipophilic opioid. Its pharmacological profile is unique and high-yield for NEET-PG: 1. **Why Option A is Correct:** Buprenorphine acts as a **partial agonist at the $\mu$ (mu) opioid receptor**. It has a very high affinity for the receptor but low intrinsic activity. This results in a "ceiling effect" for respiratory depression, making it safer in overdose compared to full agonists like morphine. It also has a slow dissociation rate from the $\mu$ receptor, leading to a long duration of action. 2. **Why Options B & D are Incorrect:** Buprenorphine is actually an **antagonist at the $\kappa$ (kappa) receptor**, not an agonist. This $\kappa$-antagonism is thought to contribute to its antidepressant effects and lack of dysphoria. 3. **Why Option C is Incorrect:** Full $\mu$ agonists (e.g., Morphine, Fentanyl, Methadone) have high intrinsic activity without a ceiling effect. Buprenorphine can actually antagonize the effects of full agonists if administered concurrently, potentially precipitating withdrawal in opioid-dependent individuals. **High-Yield Clinical Pearls for NEET-PG:** * **Mixed Action:** It is a $\mu$ partial agonist and $\kappa$ antagonist. * **Ceiling Effect:** Increasing the dose beyond a certain point does not increase the analgesic effect or respiratory depression. * **Clinical Use:** Used in opioid substitution therapy (detoxification) and chronic pain management. * **Naloxone Interaction:** Because it binds so tightly to $\mu$ receptors, buprenorphine-induced respiratory depression is difficult to reverse with standard doses of Naloxone.

Question 169: Which of the following is an anxiolytic drug?

A. Lithium
B. Oxazepam (Correct Answer)
C. Desipramine
D. Chlorpromazine

Explanation: **Explanation:** **Oxazepam** is the correct answer because it belongs to the **Benzodiazepine (BZD)** class of drugs. Benzodiazepines act as positive allosteric modulators of the **GABA-A receptor**, increasing the frequency of chloride channel opening. This leads to hyperpolarization of neurons, resulting in sedative, hypnotic, and potent **anxiolytic** (anxiety-reducing) effects. **Analysis of Incorrect Options:** * **Lithium (A):** This is the gold-standard **mood stabilizer** used primarily in the treatment of Bipolar Affective Disorder (BPAD) and acute mania. It has no primary anxiolytic properties. * **Desipramine (C):** This is a **Tricyclic Antidepressant (TCA)**, specifically a selective norepinephrine reuptake inhibitor. While some antidepressants are used for chronic anxiety disorders, Desipramine is primarily classified as an antidepressant. * **Chlorpromazine (D):** This is a typical **Antipsychotic** (low-potency Neuroleptic) belonging to the Phenothiazine class. It works by blocking D2 receptors and is used to treat schizophrenia and psychosis. **NEET-PG High-Yield Pearls:** 1. **Metabolism:** Oxazepam, along with Lorazepam and Temazepam (mnemonic: **LOT**), undergoes direct glucuronidation and does not form active metabolites. This makes them the **drugs of choice in elderly patients or those with liver failure**. 2. **Short-acting:** Oxazepam has a slow onset and short duration of action, making it suitable for treating insomnia characterized by difficulty staying asleep or for managing alcohol withdrawal. 3. **Antidote:** In cases of Benzodiazepine overdose, the specific antagonist is **Flumazenil**.

Question 170: A patient with recent-onset primary generalized epilepsy develops a drug reaction with a skin rash due to Phenytoin sodium. What is the most appropriate course of action?

A. Shift to clonazepam
B. Restart phenytoin sodium after 2 weeks
C. Shift to sodium valproate (Correct Answer)
D. Shift to Ethosuximide

Explanation: ### Explanation **1. Why Sodium Valproate is the Correct Choice:** The patient is suffering from **Primary Generalized Epilepsy (PGE)** and has developed a hypersensitivity reaction (skin rash) to Phenytoin. In PGE, which includes generalized tonic-clonic seizures (GTCS), absence seizures, and myoclonic seizures, **Sodium Valproate** is the drug of choice (DOC) because of its broad-spectrum activity. Since Phenytoin caused a rash (suggesting a potential progression to Stevens-Johnson Syndrome), it must be discontinued immediately and replaced with an equally effective broad-spectrum agent that does not share the same chemical structure. **2. Analysis of Incorrect Options:** * **Option A (Clonazepam):** While it has anticonvulsant properties, it is primarily used as an adjunctive treatment or for specific types like myoclonic seizures. It is not a first-line monotherapy for generalized epilepsy due to the development of tolerance and sedative side effects. * **Option B (Restart Phenytoin):** This is dangerous. A skin rash induced by aromatic anticonvulsants (Phenytoin, Carbamazepine, Phenobarbital) can be a precursor to life-threatening conditions like **SJS or TEN**. Re-challenging the patient with the same drug is contraindicated. * **Option D (Ethosuximide):** This drug is the DOC for **Absence Seizures only**. It is ineffective against GTCS, which is a major component of primary generalized epilepsy. **3. Clinical Pearls for NEET-PG:** * **Broad-Spectrum AEDs:** Valproate, Levetiracetam, Topiramate, and Zonisamide are effective against both focal and generalized seizures. * **Narrow-Spectrum AEDs:** Phenytoin and Carbamazepine can actually **worsen** certain generalized seizures like absence and myoclonic seizures. * **HLA-B*1502:** Always remember the association between this allele and Carbamazepine/Phenytoin-induced SJS, especially in Asian populations. * **Alternative:** If Valproate is contraindicated (e.g., in a woman of childbearing age), **Levetiracetam** or **Lamotrigine** (titrated slowly) are preferred alternatives.

Practice by Chapter

General Anesthetics

Practice Questions

Local Anesthetics

Practice Questions

Sedative-Hypnotics

Practice Questions

Antiepileptic Drugs

Practice Questions

Antiparkinsonian Drugs

Practice Questions

Opioid Analgesics

Practice Questions

Drugs of Abuse and Addiction

Practice Questions

Psychostimulants

Practice Questions

Hallucinogens

Practice Questions

CNS Stimulants and Cognitive Enhancers

Practice Questions

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