Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 11: What is the preferred drug for alcohol withdrawal seizures?

A. Diazepam (Correct Answer)
B. Valproate
C. Phenobarbitone
D. Carbamazepine

Explanation: **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.

Question 12: Which of the following statements is false regarding phenobarbitone?

A. Acts primarily at the GABA-BZD receptor-chloride ion channel complex
B. Crosses the placenta
C. May precipitate porphyria
D. Has no significant interaction with warfarin (Correct Answer)

Explanation: **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 13: What is the drug of choice for Huntington's chorea?

A. Haloperidol
B. Chlorpromazine
C. Tetrabenazine (Correct Answer)
D. Donepezil

Explanation: **Explanation:** **Huntington’s Chorea** is a neurodegenerative disorder characterized by a loss of GABAergic neurons in the striatum, leading to a functional **excess of dopamine**. This dopaminergic overactivity results in involuntary, jerky movements (chorea). **Why Tetrabenazine is the Correct Answer:** Tetrabenazine is the **Drug of Choice (DOC)** for managing chorea. Its mechanism of action involves the reversible inhibition of **Vesicular Monoamine Transporter 2 (VMAT2)**. By blocking VMAT2, it prevents the packaging of dopamine into presynaptic vesicles, leading to the depletion of dopamine stores in the nerve terminals. This reduction in available dopamine effectively suppresses the hyperkinetic movements. (Note: **Deutetrabenazine**, a deuterated form with a longer half-life, is also FDA-approved and often preferred for its better side-effect profile). **Analysis of Incorrect Options:** * **A & B (Haloperidol & Chlorpromazine):** These are typical antipsychotics that act as **D2 receptor antagonists**. While they can suppress chorea, they are considered second-line due to a higher risk of severe extrapyramidal side effects (EPS) and tardive dyskinesia. * **D (Donepezil):** This is an acetylcholinesterase inhibitor used in **Alzheimer’s disease**. It has no role in treating the motor symptoms of Huntington’s. **High-Yield Clinical Pearls for NEET-PG:** * **Side Effects of Tetrabenazine:** The most significant risk is **depression and suicidality**; it should be used cautiously in patients with a history of psychiatric illness. * **Reserpine:** Like tetrabenazine, it inhibits VMAT, but it is irreversible and acts peripherally, causing significant hypotension, making it less favorable. * **Genetics:** Huntington’s is an **Autosomal Dominant** disorder associated with **CAG repeats** on Chromosome 4 (Huntingtin gene), showing the phenomenon of **anticipation**.

Question 14: Which of the following antihistamines is least preferable for motion sickness?

A. Cetirizine
B. Meclizine
C. Diphenhydramine
D. Fexofenadine (Correct Answer)

Explanation: **Explanation:** To be effective in treating motion sickness, an antihistamine must cross the **blood-brain barrier (BBB)** and possess significant **anticholinergic (antimuscarinic) activity**. Motion sickness is triggered by vestibular stimulation, which sends signals to the vomiting center via histaminergic (H1) and cholinergic (M1) pathways. **Why Fexofenadine is the correct answer:** Fexofenadine is a **second-generation antihistamine**. These drugs are highly polar, have low lipid solubility, and are substrates for the P-glycoprotein efflux pump, which prevents them from crossing the BBB. Furthermore, fexofenadine lacks significant anticholinergic activity. Therefore, it cannot act on the vestibular nuclei or the vomiting center, making it ineffective for motion sickness. **Analysis of incorrect options:** * **Diphenhydramine:** A first-generation antihistamine with high lipid solubility and potent anticholinergic effects. It is highly effective for motion sickness but causes significant sedation. * **Meclizine:** A first-generation antihistamine specifically preferred for motion sickness and vertigo due to its long duration of action and less sedative profile compared to diphenhydramine. * **Cetirizine:** While technically a second-generation agent, it is a metabolite of hydroxyzine and can cause mild sedation in some patients. Although not the first choice, it has slightly more CNS penetration than fexofenadine, though fexofenadine remains the "least preferable" among the list. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC)** for motion sickness prophylaxis: **Hyoscine (Scopolamine)**, administered via a transdermal patch behind the ear. * **First-generation antihistamines** (e.g., Promethazine, Cyclizine, Dimenhydrinate) are used for motion sickness because they are **inverse agonists** at H1 receptors and block M1 receptors. * **Fexofenadine** is the active metabolite of Terfenadine and is considered the "purest" second-generation antihistamine (least sedative).

Question 15: All of the following substances can cause seizures except?

A. Quinolones
B. Tramadol
C. Chloroquine
D. Lamotrigine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Lamotrigine**. **1. Why Lamotrigine is the correct answer:** Lamotrigine is an **anti-epileptic drug (AED)** [1]. Its primary mechanism of action involves blocking voltage-gated sodium channels and inhibiting the release of excitatory neurotransmitters like glutamate [2]. Because its therapeutic purpose is to stabilize neuronal membranes and prevent repetitive firing, it is used to *treat* seizures (especially partial and generalized tonic-clonic seizures) rather than cause them [3]. **2. Why the other options are incorrect (Pro-convulsant drugs):** * **Quinolones (e.g., Ciprofloxacin):** These antibiotics inhibit GABA receptors (the brain's primary inhibitory neurotransmitter). By blocking GABAergic inhibition, they lower the seizure threshold, especially in patients with pre-existing CNS disorders or renal impairment. * **Tramadol:** This opioid analgesic inhibits the reuptake of serotonin and norepinephrine. At therapeutic or toxic doses, it significantly lowers the seizure threshold. The risk is further increased when combined with SSRIs or TCAs (Serotonin Syndrome). * **Chloroquine:** This antimalarial drug has known neurotoxic effects and can induce seizures even in patients without a prior history of epilepsy, likely due to its interference with GABAergic signaling. **3. NEET-PG High-Yield Clinical Pearls:** * **Other common drugs that lower seizure threshold:** Bupropion (antidepressant), Imipenem (carbapenem), Isoniazid (INH - due to Vitamin B6 depletion), and Theophylline. * **Lamotrigine specific:** It is a first-line drug for **Lennox-Gastaut Syndrome** and is also used as a mood stabilizer in Bipolar Disorder [3]. A critical side effect to remember for exams is **Stevens-Johnson Syndrome (SJS)**; hence, the dose must be titrated slowly. * **Isoniazid-induced seizures** are specifically treated with intravenous **Pyridoxine (Vitamin B6)**.

Question 16: The effect of thiopentone on the central nervous system is quickly terminated because of which of the following mechanisms?

A. Rapid metabolism within the central nervous system
B. Quick first-pass elimination by the liver
C. Redistribution to other tissues (Correct Answer)
D. Rapid metabolism in systemic circulation

Explanation: **Explanation:** The rapid termination of the anesthetic effect of **Thiopentone** (an ultra-short-acting barbiturate) is due to the phenomenon of **redistribution**, not metabolism. 1. **Why Option C is Correct:** Thiopentone is highly lipid-soluble. Upon intravenous injection, it rapidly crosses the blood-brain barrier and reaches peak concentrations in the brain (a highly perfused organ) within seconds, inducing anesthesia. However, as the drug continues to circulate, it moves down its concentration gradient from the brain back into the blood and is then **redistributed** to less perfused but larger-volume tissues, such as skeletal muscle and eventually adipose tissue. This drop in brain concentration leads to a quick recovery of consciousness (usually within 5–10 minutes), even though the drug is still present in the body. 2. **Why Other Options are Incorrect:** * **Options A & D:** Thiopentone is not metabolized in the CNS or systemic circulation. It undergoes slow hepatic metabolism (approx. 10–15% per hour). * **Option B:** While Thiopentone is metabolized by the liver, this process is far too slow to account for the rapid awakening seen after a single bolus dose. **High-Yield NEET-PG Pearls:** * **Context:** Redistribution is the characteristic mechanism for terminating the action of all highly lipid-soluble drugs given intravenously (e.g., Thiopentone, Propofol, Fentanyl). * **Cumulative Effect:** If Thiopentone is given via repeated doses or continuous infusion, the storage sites (muscle/fat) become saturated. At this point, the duration of action depends on metabolism rather than redistribution, leading to a prolonged recovery time (hangover effect). * **Context-Sensitive Half-life:** This is why Thiopentone has a long context-sensitive half-life compared to Propofol, making it unsuitable for maintaining anesthesia.

Question 17: Which of the following muscle relaxants is contraindicated in muscular dystrophy?

A. Quinine
B. Succinylcholine (Correct Answer)
C. Dantrolene
D. All of the above

Explanation: ### Explanation **Correct Option: B. Succinylcholine** **Mechanism & Contraindication:** Succinylcholine is a **depolarizing neuromuscular blocker**. In patients with muscular dystrophy (especially Duchenne and Becker types), the muscle cell membranes are fragile due to the absence or deficiency of dystrophin. When succinylcholine causes prolonged depolarization, it can trigger a massive, sudden release of intracellular potassium into the bloodstream (**Hyperkalemia**). This occurs because of the upregulation of extrajunctional nicotinic receptors. This acute hyperkalemia can lead to life-threatening cardiac arrhythmias and cardiac arrest. Additionally, these patients are at a higher risk for **Rhabdomyolysis** and **Malignant Hyperthermia** when exposed to succinylcholine. **Analysis of Incorrect Options:** * **A. Quinine:** While quinine is used for nocturnal muscle cramps and has some neuromuscular effects, it is not specifically contraindicated in muscular dystrophy; it is more famously avoided in Myasthenia Gravis as it can exacerbate weakness. * **C. Dantrolene:** Dantrolene is a direct-acting muscle relaxant that inhibits calcium release from the sarcoplasmic reticulum. It is actually the **drug of choice** for treating Malignant Hyperthermia, a condition associated with muscular dystrophy, and is not contraindicated. * **D. All of the above:** Incorrect, as only succinylcholine poses the specific risk of hyperkalemic cardiac arrest in this population. **High-Yield Clinical Pearls for NEET-PG:** * **Black Box Warning:** The FDA has a black box warning for succinylcholine regarding its use in children due to the risk of undiagnosed skeletal muscle myopathy. * **Other Contraindications:** Succinylcholine should also be avoided in patients with major burns, massive trauma, or prolonged immobilization (usually 24–72 hours post-injury) due to the same risk of hyperkalemia. * **Treatment of Toxicity:** If succinylcholine triggers Malignant Hyperthermia, the immediate treatment is **Dantrolene**.

Question 18: Prolonged use of which of the following anticonvulsants can produce weight loss?

A. Gabapentin
B. Oxcarbazepine
C. Topiramate (Correct Answer)
D. Valproic acid

Explanation: **Explanation:** **Topiramate** is a broad-spectrum antiepileptic drug known for its unique side effect profile. Unlike many anticonvulsants that cause weight gain, Topiramate is characteristically associated with **weight loss** and anorexia. This occurs due to its inhibitory effect on carbonic anhydrase enzymes and its influence on hypothalamic feeding centers, leading to reduced appetite and altered taste (dysgeusia). This property makes it a preferred choice for obese patients with epilepsy or migraine and has led to its FDA approval (in combination with phentermine) for chronic weight management. **Analysis of Incorrect Options:** * **Valproic acid:** This is a classic "weight gain" drug. It increases appetite and can lead to significant weight gain and metabolic syndrome, often making it a poor choice for patients with PCOS. * **Gabapentin:** Frequently associated with weight gain and peripheral edema. It is more commonly used for neuropathic pain than as a primary anticonvulsant. * **Oxcarbazepine:** Generally considered weight-neutral, though its most significant electrolyte abnormality is **hyponatremia** (more common than with carbamazepine). **NEET-PG High-Yield Pearls:** * **Topiramate Side Effects:** Remember the mnemonic "M-O-U-T-H": **M**etabolic acidosis (due to carbonic anhydrase inhibition), **O**cular effects (acute myopia/secondary angle-closure glaucoma), **U**rolithiasis (kidney stones), **T**eratogenicity (cleft lip/palate), and **H**ypohidrosis (reduced sweating/hyperthermia). * **Cognitive Dullings:** Topiramate is often nicknamed "Stupimax" because it can cause word-finding difficulties and cognitive slowing. * **Zonisamide** is another anticonvulsant that also causes weight loss and kidney stones.

Question 19: Fosphenytoin is different from phenytoin as:

A. It is less damaging to the intima on intravenous administration.
B. It can be injected in a drip of glucose solution.
C. It is a prodrug of phenytoin.
D. All the above. (Correct Answer)

Explanation: Fosphenytoin is a water-soluble phosphate ester **prodrug** of phenytoin, developed specifically to overcome the pharmaceutical limitations of parenteral phenytoin [1].1. **Why Option D is correct:** * **Option A (Less Intimal Damage):** Phenytoin is highly alkaline (pH ~12) and requires propylene glycol as a solvent, which causes severe local irritation, pain, and "Purple Glove Syndrome" (extensive limb ischemia). Fosphenytoin has a near-physiological pH (8.6–9.0) and is aqueous-based, making it significantly less damaging to the venous intima [1]. * **Option B (Compatibility with Glucose):** Phenytoin is highly insoluble and precipitates immediately if added to glucose solutions (it must only be used with Normal Saline). Fosphenytoin is highly water-soluble and remains stable in both **Dextrose (5%) and Normal Saline**, allowing for more flexible intravenous administration [1]. * **Option C (Prodrug Status):** Fosphenytoin is pharmacologically inactive until it is converted to phenytoin by **phosphatases** in the blood and liver (conversion half-life is ~8–15 minutes).**High-Yield Clinical Pearls for NEET-PG:** * **Dosing:** Fosphenytoin is prescribed in **Phenytoin Equivalents (PE)**. 1.5 mg of fosphenytoin = 1 mg of phenytoin.* **Infusion Rate:** It can be infused faster than phenytoin (up to **150 mg PE/min** vs. 50 mg/min for phenytoin), allowing for quicker achievement of therapeutic levels in Status Epilepticus.* **Monitoring:** While local complications are fewer, systemic side effects like **cardiac arrhythmias and hypotension** can still occur; ECG monitoring is mandatory during infusion.

Question 20: Which of the following is an exclusive 5-HT 1F receptor agonist approved by FDA for treatment of acute migraine attack?

A. Rimegepant
B. Lasmiditan (Correct Answer)
C. Eletriptan
D. Dosulepin

Explanation: **Explanation:** **Lasmiditan** is the correct answer. It is a first-in-class **selective 5-HT 1F receptor agonist** approved by the FDA for the acute treatment of migraine with or without aura. Unlike the "Triptans," which act on 5-HT 1B/1D receptors [1], Lasmiditan lacks vasoconstrictive properties because 5-HT 1F receptors are located on trigeminal nerve endings and not on blood vessels. This makes it a safer alternative for patients with cardiovascular contraindications. **Analysis of Incorrect Options:** * **A. Rimegepant:** This is a small-molecule **CGRP receptor antagonist** (Gepant). While used for acute migraine, its mechanism involves blocking the CGRP receptor rather than stimulating 5-HT receptors [3]. * **C. Eletriptan:** This is a standard **5-HT 1B/1D agonist** [1]. Its activation of 1B receptors causes cranial vasoconstriction, which is why triptans are contraindicated in patients with ischemic heart disease or uncontrolled hypertension [2]. * **D. Dosulepin:** This is a **Tricyclic Antidepressant (TCA)**. While TCAs (like Amitriptyline) are used for migraine *prophylaxis*, they are not used for acute attacks and do not act as selective 5-HT 1F agonists. **High-Yield Clinical Pearls for NEET-PG:** * **"Ditans" vs. "Triptans":** Ditans (Lasmiditan) = No vasoconstriction (Safe in CAD). Triptans = Vasoconstriction (Unsafe in CAD). * **Side Effect:** The most common side effect of Lasmiditan is **dizziness/somnolence**; patients are advised not to drive for 8 hours after taking a dose. * **Site of Action:** Lasmiditan acts centrally and peripherally to inhibit the release of neuropeptides and neurotransmitters involved in migraine pain signaling [3].

Practice by Chapter

General Anesthetics

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Local Anesthetics

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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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