Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 451: Tiazinadine is:

A. GABAA agonist
B. Central alpha2 agonist (Correct Answer)
C. Anti-muscarinic agent
D. GABAB agonist

Explanation: **Explanation:** **Tizanidine** is a centrally acting skeletal muscle relaxant. Its primary mechanism of action is as a **selective alpha-2 ($\alpha_2$) adrenergic agonist**. It acts predominantly in the spinal cord by stimulating presynaptic $\alpha_2$ receptors. This leads to a reduction in the release of excitatory amino acids (like glutamate and aspartate) from spinal interneurons, thereby inhibiting spinal polysynaptic reflex arcs and reducing muscle spasticity. **Analysis of Options:** * **Option A (GABA-A agonist):** This describes the mechanism of **Benzodiazepines** (e.g., Diazepam), which increase the frequency of chloride channel opening to enhance CNS inhibition. * **Option C (Anti-muscarinic agent):** These drugs (e.g., Atropine, Trihexyphenidyl) block acetylcholine at muscarinic receptors. While used in Parkinsonism, they are not the mechanism for Tizanidine. * **Option D (GABA-B agonist):** This is the mechanism of **Baclofen**, which acts as a metabotropic GABA-B agonist to cause hyperpolarization and muscle relaxation. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Use:** Primarily used for spasticity associated with cerebral palsy, multiple sclerosis, and spinal cord injuries. * **Advantage:** Unlike Baclofen or Diazepam, Tizanidine causes **less muscle weakness**, making it preferred when maintaining motor strength is necessary. * **Side Effects:** Common side effects include **hypotension** (due to its $\alpha_2$ agonist nature, similar to clonidine), xerostomia (dry mouth), and somnolence. * **Metabolism:** It is metabolized by **CYP1A2**; therefore, it is contraindicated with potent CYP1A2 inhibitors like Ciprofloxacin or Fluvoxamine.

Question 452: Use of valproate during pregnancy may result in which of the following adverse outcomes?

A. Mental retardation
B. Respiratory depression
C. Hydantoin syndrome
D. Neural tube defect (Correct Answer)

Explanation: **Explanation:** **Sodium Valproate** is a broad-spectrum antiepileptic drug, but it is highly teratogenic. The correct answer is **Neural Tube Defects (NTDs)**, specifically **spina bifida**. 1. **Mechanism of Teratogenicity:** Valproate causes NTDs in approximately 1–2% of exposed pregnancies. The primary mechanism is the inhibition of **histone deacetylase (HDAC)** and interference with **folate metabolism**, which are critical for the proper closure of the neural tube during the first trimester. 2. **Why other options are incorrect:** * **Mental Retardation:** While valproate exposure is linked to lower IQ and autism spectrum disorders (Fetal Valproate Syndrome), "Neural Tube Defect" is the classic, pathognomonic structural malformation tested in exams. * **Respiratory Depression:** This is an acute neonatal complication of maternal opioid or benzodiazepine use, not a structural teratogenic effect of valproate. * **Hydantoin Syndrome:** This is specifically associated with **Phenytoin** use during pregnancy, characterized by craniofacial dysmorphism, hypoplastic phalanges, and growth retardation. **High-Yield Clinical Pearls for NEET-PG:** * **Fetal Valproate Syndrome:** Includes NTDs, atrial septal defects, cleft lip/palate, and "cupid’s bow" upper lip. * **Prevention:** High-dose folic acid (4–5 mg/day) is recommended pre-conceptionally for women on antiepileptics, though it may not fully neutralize valproate's risk. * **Drug of Choice in Pregnancy:** **Lamotrigine** and **Levetiracetam** are generally considered the safest options. * **Avoidance:** Valproate should be avoided in females of childbearing age unless no other drug is effective for their seizure type (e.g., Myoclonic seizures).

Question 453: What electrolyte imbalance is commonly caused by carbamazepine in the elderly?

A. Hypernatremia
B. Hyponatremia (Correct Answer)
C. Hyperkalemia
D. Hypokalemia

Explanation: **Explanation:** **1. Why Hyponatremia is Correct:** Carbamazepine is a well-known cause of **SIADH-like syndrome** (Syndrome of Inappropriate Antidiuretic Hormone). It acts by increasing the sensitivity of the renal tubules to ADH and potentially stimulating the central release of ADH. This leads to free water retention and dilutional hyponatremia. This side effect is particularly prevalent in the elderly (up to 10–15% of patients) due to age-related changes in water homeostasis and the frequent co-administration of other drugs like diuretics. **2. Why Other Options are Incorrect:** * **Hypernatremia (A):** Carbamazepine causes water retention, not water loss or sodium excess. Hypernatremia is more commonly associated with Lithium (via Nephrogenic Diabetes Insipidus). * **Hyperkalemia (C) & Hypokalemia (D):** Carbamazepine does not have a primary or direct effect on potassium excretion or shift. While profound hyponatremia can sometimes be seen alongside other electrolyte shifts in complex clinical scenarios, potassium imbalance is not a characteristic side effect of this drug. **3. High-Yield Clinical Pearls for NEET-PG:** * **Oxcarbazepine:** A prodrug of carbamazepine, it is actually **more likely** to cause hyponatremia than carbamazepine itself. * **Monitoring:** Always check serum sodium levels if an elderly patient on carbamazepine presents with new-onset confusion, lethargy, or seizures (which may be due to hyponatremia rather than epilepsy). * **Other Key Side Effects:** Stevens-Johnson Syndrome (associated with **HLA-B*1502** allele), Aplastic anemia, and Enzyme Induction (CYP3A4). * **Drug of Choice:** Carbamazepine remains the DOC for **Trigeminal Neuralgia**.

Question 454: Which of the following antiepileptics are cytochrome P450 enzyme inducers?

A. Phenytoin
B. Phenobarbital
C. Carbamazepine
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (All of the above)**. This question tests the knowledge of drug metabolism and the interaction of antiepileptic drugs (AEDs) with the hepatic microsomal enzyme system. **1. Understanding the Concept:** Cytochrome P450 (CYP450) enzyme inducers are substances that increase the synthesis and activity of hepatic enzymes. When an inducer is administered, it accelerates the metabolism of both itself (auto-induction) and other co-administered drugs. This leads to decreased plasma concentrations and reduced therapeutic efficacy of drugs like oral contraceptives, warfarin, and theophylline. **2. Analysis of Options:** * **Phenytoin:** A potent inducer of CYP3A4 and CYP2C subfamilies. It is notorious for causing numerous drug interactions and follows zero-order kinetics at high doses. * **Phenobarbital:** One of the oldest and most powerful enzyme inducers. It increases the synthesis of glucuronyl transferase and CYP enzymes, often used clinically to treat neonatal jaundice (by inducing bilirubin conjugation). * **Carbamazepine:** A classic enzyme inducer that notably exhibits **auto-induction**, meaning it induces its own metabolism, requiring dosage adjustments after the first few weeks of therapy. **3. Clinical Pearls for NEET-PG:** * **Mnemonic for Inducers:** "**G**P **S**ell **C**ar **P**hone **O**ut" (**G**riseofulvin, **P**henytoin, **S**moking, **C**arbamazepine, **P**henobarbital, **O**xcarbazepine). * **Exceptions:** **Valproate** is a significant exception among older AEDs; it is a **CYP enzyme inhibitor**, not an inducer. * **Clinical Impact:** If a patient on Warfarin starts Carbamazepine, their INR will drop, increasing the risk of thrombosis. Conversely, stopping an inducer without adjusting other medications can lead to toxicity of the co-administered drug.

Question 455: What is the drug of choice for acute severe migraine?

A. Ergotamine
B. Sumatriptan (Correct Answer)
C. Dihydroergotamine
D. Propranolol

Explanation: **Explanation:** **1. Why Sumatriptan is the Correct Answer:** Sumatriptan, a selective **5-HT$_{1B/1D}$ receptor agonist**, is the drug of choice (DOC) for **acute severe migraine** attacks. Its mechanism involves: * **Vasoconstriction:** Stimulating 5-HT$_{1B}$ receptors on intracranial blood vessels to reverse vasodilation. * **Neural Inhibition:** Stimulating 5-HT$_{1D}$ receptors on trigeminal nerve endings to inhibit the release of pro-inflammatory neuropeptides (like CGRP and Substance P). Sumatriptan is preferred due to its rapid onset (especially via subcutaneous or nasal routes) and better side-effect profile compared to ergots. **2. Analysis of Incorrect Options:** * **Ergotamine (A) & Dihydroergotamine (C):** These are non-selective 5-HT agonists. While effective for severe migraine, they have poor oral bioavailability, higher rates of nausea/vomiting, and a risk of prolonged vasoconstriction (ergotism). They are now considered second-line to triptans. * **Propranolol (D):** This is a beta-blocker used for the **prophylaxis** (prevention) of migraine, not for treating an acute attack. It is the DOC for chronic migraine prevention. **3. High-Yield Clinical Pearls for NEET-PG:** * **Triptan Contraindications:** Avoid in patients with Ischemic Heart Disease (IHD), Prinzmetal angina, or uncontrolled hypertension due to coronary vasoconstriction. * **Triptan Sensation:** Patients may experience chest tightness or "heaviness," which is usually non-cardiac but must be monitored. * **Status Migrainosus:** The DOC for a migraine attack lasting >72 hours is **IV Dihydroergotamine** or **IV Dexamethasone**. * **Mild-Moderate Migraine:** The first-line treatment is typically NSAIDs (e.g., Naproxen or Aspirin).

Question 456: What is the antidote for ethylene glycol poisoning?

A. Methyl violet
B. Fluconazole
C. Fomepizole (Correct Answer)
D. Ethyl alcohol

Explanation: **Explanation:** Ethylene glycol poisoning is a medical emergency characterized by high anion gap metabolic acidosis and acute renal failure. The toxicity is not caused by the parent compound itself, but by its metabolites (**glycolic acid** and **oxalic acid**). **Why Fomepizole is correct:** The rate-limiting step in the metabolism of ethylene glycol is its conversion to glycoaldehyde by the enzyme **Alcohol Dehydrogenase (ADH)**. **Fomepizole** is a potent competitive inhibitor of ADH. By blocking this enzyme, it prevents the formation of toxic metabolites, allowing the parent compound to be excreted harmlessly by the kidneys. It is the preferred antidote due to its predictable kinetics and lack of CNS depression. **Analysis of Incorrect Options:** * **Methyl violet (A):** This is a histological stain and antifungal agent; it has no role in toxicology. * **Fluconazole (B):** An antifungal medication that inhibits CYP450 enzymes, not ADH. * **Ethyl alcohol (D):** While Ethanol also acts as an antidote by competing for ADH (it has a higher affinity for the enzyme than ethylene glycol), it is no longer the first-line choice. It requires constant IV infusion, monitoring of blood ethanol levels, and causes significant CNS depression and hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Renal Failure:** Oxalic acid precipitates with calcium to form **calcium oxalate crystals** (envelope-shaped) in the renal tubules. * **Alternative Antidote:** Ethanol is used only if Fomepizole is unavailable. * **Cofactor Therapy:** Pyridoxine (B6) and Thiamine (B1) are often administered to shunt metabolism toward less toxic pathways. * **Methanol Poisoning:** Fomepizole is also the specific antidote for Methanol poisoning.

Question 457: Which of the following is an inverse agonist of benzodiazepines (BZD)?

A. Beta-carboline (Correct Answer)
B. Bicuculine
C. Baclofen
D. Diazepam

Explanation: ### Explanation **Correct Option: A. Beta-carboline** Benzodiazepines (BZDs) act as **positive allosteric modulators** at the GABA-A receptor, increasing the frequency of chloride channel opening. An **inverse agonist**, such as **Beta-carboline**, binds to the same BZD receptor site but produces the opposite pharmacological effect. While BZDs are anxiolytic and anticonvulsant, Beta-carbolines are **anxiogenic** (induce anxiety) and **pro-convulsant**. **Analysis of Incorrect Options:** * **B. Bicuculline:** This is a **competitive antagonist** at the GABA binding site of the GABA-A receptor. It does not act on the BZD site. * **C. Baclofen:** This is a selective **GABA-B receptor agonist**. It is used clinically as a centrally acting muscle relaxant and acts via G-protein coupled receptors, not the ionotropic GABA-A receptor. * **D. Diazepam:** This is a classic **BZD agonist**. It facilitates GABA action, leading to sedation, hypnosis, and muscle relaxation. **High-Yield Clinical Pearls for NEET-PG:** * **Flumazenil:** It is a **pure antagonist** at the BZD site. It blocks the actions of both BZD agonists (Diazepam) and inverse agonists (Beta-carboline). It is the drug of choice for BZD overdose. * **GABA-A Receptor:** A pentameric ligand-gated ion channel ($Cl^-$). BZDs increase the **frequency** of opening, while Barbiturates increase the **duration** of opening. * **Z-drugs (Zolpidem, Zaleplon):** These are non-benzodiazepine hypnotics that act on the $\alpha_1$ subunit of the BZD receptor.

Question 458: Which of the following is NOT a side effect of amitriptyline?

A. Constipation
B. Fine tremors
C. Weight loss (Correct Answer)
D. Dry mouth

Explanation: **Explanation:** Amitriptyline is a **Tricyclic Antidepressant (TCA)** that acts by inhibiting the reuptake of Serotonin and Norepinephrine. However, its side effect profile is largely determined by its lack of selectivity, as it also blocks Muscarinic (M1), Histaminergic (H1), and Alpha-adrenergic (α1) receptors. **Why Weight Loss is the Correct Answer:** Amitriptyline is notorious for causing **Weight Gain**, not weight loss. This occurs primarily due to its potent **H1-receptor antagonism**, which increases appetite and slows metabolism. In clinical practice, it is often avoided in obese patients but may be used in patients with insomnia or low appetite. **Analysis of Incorrect Options:** * **Constipation & Dry Mouth (Options A & D):** These are classic **Anticholinergic (Antimuscarinic)** side effects. TCAs block M1 receptors, leading to the "dry" symptoms: dry mouth (xerostomia), constipation, urinary retention, and blurred vision. * **Fine Tremors (Option B):** This is a common side effect resulting from increased **noradrenergic activity** in the CNS and peripheral nervous system. **High-Yield Clinical Pearls for NEET-PG:** * **The "3 Cs" of TCA Overdose:** Coma, Convulsions, and **Cardiotoxicity** (due to Sodium channel blockade leading to QRS prolongation). * **Antidote for Cardiotoxicity:** Sodium Bicarbonate (to stabilize the cardiac membrane). * **Other Side Effects:** Orthostatic hypotension (α1 blockade) and sedation (H1 blockade). * **Contraindication:** Recent Myocardial Infarction and Narrow-angle glaucoma.

Question 459: Pitolisant is approved for the treatment of which of the following conditions?

A. Tenosynovial giant cell tumor
B. Excessive daytime sleepiness in patients with narcolepsy (Correct Answer)
C. Polyneuropathy due to transthyretin amyloidosis
D. Primary myelofibrosis

Explanation: **Explanation:** **Pitolisant** is a first-in-class selective **Histamine H3-receptor antagonist/inverse agonist**. By blocking H3 autoreceptors in the brain, it increases the synthesis and release of histamine, a key neurotransmitter that promotes wakefulness. It is FDA-approved for the treatment of **excessive daytime sleepiness (EDS)** and **cataplexy** in adult patients with **narcolepsy**. Unlike other wake-promoting agents (e.g., Modafinil), Pitolisant is not a scheduled controlled substance, as it lacks significant abuse potential. **Analysis of Incorrect Options:** * **Option A (Tenosynovial giant cell tumor):** This is treated with **Pexidartinib**, a CSF1 receptor inhibitor. * **Option C (Polyneuropathy due to transthyretin amyloidosis):** This is treated with agents like **Patisiran** (RNA interference) or **Tafamidis** (TTR stabilizer). * **Option D (Primary myelofibrosis):** This is typically managed with JAK inhibitors like **Ruxolitinib** or **Fedratinib**. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** H3-receptor inverse agonist (increases histamine levels in the CNS). * **Unique Feature:** It is the first non-scheduled drug for narcolepsy (low potential for addiction). * **Contraindications:** Severe hepatic impairment. It can also prolong the **QT interval**, so caution is required with other pro-arrhythmic drugs. * **Drug Interactions:** It is a CYP2D6 substrate and a weak inducer of CYP3A4, which may reduce the efficacy of hormonal contraceptives.

Question 460: What is the drug of choice for alcohol withdrawal?

A. TFP
B. Chlormethazole
C. Chlordiazepoxide (Correct Answer)
D. Buspirone

Explanation: ### Explanation **Correct Option: C. Chlordiazepoxide** The primary goal in managing alcohol withdrawal is to prevent seizures, delirium tremens, and autonomic hyperactivity. Alcohol is a CNS depressant that chronically enhances GABAergic tone. Abrupt cessation leads to a "hyperexcitable" state. **Benzodiazepines (BZDs)** are the drugs of choice because they show **cross-tolerance** with alcohol, acting on the same GABA-A receptors to provide a "tapered" sedative effect. * **Chlordiazepoxide** and **Diazepam** are preferred due to their **long half-lives** and active metabolites, which provide a smoother "self-tapering" effect and superior protection against withdrawal seizures. **Analysis of Incorrect Options:** * **A. TFP (Trifluoperazine):** This is a typical antipsychotic. Antipsychotics should generally be avoided as monotherapy in withdrawal because they **lower the seizure threshold**, increasing the risk of convulsions. * **B. Chlormethiazole:** While historically used for alcohol withdrawal (especially in the UK/Europe), it has a high risk of respiratory depression and a narrow therapeutic index. It is not the first-line choice globally compared to BZDs. * **D. Buspirone:** This is a non-benzodiazepine anxiolytic (5-HT1A agonist). It lacks cross-tolerance with alcohol and has no anticonvulsant properties, making it ineffective for acute withdrawal. **High-Yield Clinical Pearls for NEET-PG:** * **Liver Disease Exception:** If the patient has advanced cirrhosis or liver failure, use **LOT** drugs (**L**orazepam, **O**xazepam, **T**emazepam) as they undergo direct glucuronidation and do not rely on oxidative liver metabolism. * **Wernicke’s Encephalopathy:** Always administer **Thiamine (B1)** before or with Glucose to prevent precipitating Wernicke’s. * **Disulfiram:** Used for **aversion therapy** (maintenance of abstinence), never during the acute withdrawal phase. * **Acamprosate:** Used to reduce cravings by modulating NMDA receptors.

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