Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 561: Renal excretion of lithium is reduced by which of the following medications?

A. Furosemide
B. Indomethacin
C. Hydrochlorothiazide
D. All of the above (Correct Answer)

Explanation: **Explanation:** The renal handling of **Lithium** is closely linked to sodium balance because the kidney (specifically the proximal tubule) treats lithium ions similarly to sodium ions. Approximately 80% of filtered lithium is reabsorbed in the proximal tubule. **Mechanism of Interactions:** 1. **Thiazide Diuretics (Hydrochlorothiazide):** These are the most notorious for causing lithium toxicity. By inhibiting sodium reabsorption in the distal tubule, they cause natriuresis (sodium loss). The body compensates by increasing sodium and lithium reabsorption in the **proximal tubule**, leading to a 25–40% increase in serum lithium levels. 2. **NSAIDs (Indomethacin):** Prostaglandins maintain renal blood flow and promote sodium excretion. NSAIDs inhibit prostaglandin synthesis, leading to reduced renal perfusion and increased proximal tubular reabsorption of sodium and lithium. 3. **Loop Diuretics (Furosemide):** While traditionally thought to have a lesser effect than thiazides, loop diuretics can also reduce lithium clearance through compensatory proximal reabsorption due to volume depletion. **Why "All of the above" is correct:** All three classes of drugs decrease the renal clearance of lithium, thereby increasing its plasma concentration and the risk of toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Safe Antihypertensives:** Calcium Channel Blockers (like Amlodipine) and Centrally acting drugs (like Methyldopa) are generally safer with lithium. * **Drugs to Avoid:** ACE inhibitors and ARBs also significantly increase lithium levels. * **The "Exception" Diuretic:** **Amiloride** is the drug of choice for treating Lithium-induced Nephrogenic Diabetes Insipidus because it blocks the ENaC channels through which lithium enters the collecting duct cells. * **Osmotic Diuretics & Acetazolamide:** These actually *increase* lithium excretion (decreasing levels).

Question 562: Moclobemide is:

A. SSRI
B. Antipsychotic drug
C. MAO inhibitor (Correct Answer)
D. Tricyclic antidepressant

Explanation: **Explanation:** **Moclobemide** is a **Reversible Inhibitor of MAO-A (RIMA)**. Monoamine oxidase (MAO) is an enzyme responsible for the oxidative deamination of neurotransmitters like norepinephrine, serotonin, and dopamine. 1. **Why Option C is Correct:** MAO inhibitors are classified into non-selective (Phenelzine) and selective inhibitors. Moclobemide selectively inhibits **MAO-A**, the enzyme primarily responsible for breaking down serotonin and norepinephrine. Because it is "reversible," it can be displaced by high concentrations of tyramine, significantly reducing the risk of the "Cheese Reaction" (hypertensive crisis) compared to older, irreversible MAOIs. 2. **Why Other Options are Incorrect:** * **Option A (SSRI):** Selective Serotonin Reuptake Inhibitors (e.g., Fluoxetine, Sertraline) work by inhibiting the SERT transporter, not the MAO enzyme. * **Option B (Antipsychotic):** These drugs (e.g., Haloperidol, Risperidone) primarily act by blocking Dopamine (D2) receptors. * **Option D (Tricyclic Antidepressant):** TCAs (e.g., Amitriptyline, Imipramine) act by inhibiting the reuptake of both Serotonin and Norepinephrine but do not inhibit the MAO enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **RIMA Advantage:** Moclobemide does not require the strict "low-tyramine diet" associated with classical MAOIs. * **Serotonin Syndrome:** Never combine Moclobemide with SSRIs or Pethidine, as this can lead to a fatal increase in synaptic serotonin. * **Drug of Choice:** While not first-line for depression today, it is useful in patients who do not tolerate the anticholinergic side effects of TCAs.

Question 563: All the following Antiepileptics are inducers except?

A. Valproate (Correct Answer)
B. Phenobarbitone
C. Phenytoin
D. Rifampicin

Explanation: **Explanation:** The core concept tested here is the effect of antiepileptic drugs (AEDs) on the **Cytochrome P450 (CYP450) enzyme system**. Most traditional AEDs are enzyme inducers, whereas Valproate is the notable exception. **1. Why Valproate is the Correct Answer:** **Valproate** is a potent **enzyme inhibitor**, not an inducer. It inhibits various CYP450 isoenzymes and UDP-glucuronosyltransferase (UGT). Clinically, this is significant because Valproate increases the plasma concentration of other drugs (e.g., it inhibits the metabolism of Phenobarbital and Lamotrigine), potentially leading to toxicity. **2. Analysis of Incorrect Options (Inducers):** * **Phenobarbitone:** A classic, potent inducer of CYP1A2, 2C9, and 3A4. It increases the metabolism of drugs like oral contraceptives and warfarin. * **Phenytoin:** A strong inducer of CYP3A4. It exhibits non-linear (zero-order) kinetics at high doses, but its primary effect on other drugs is induction. * **Rifampicin:** Although primarily an antitubercular drug, it is often included in pharmacology questions as the "prototypical" potent enzyme inducer. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Inducers:** "**G**P **S**teps **P**hone **C**arefully" (**G**riseofulvin, **P**henytoin, **S**moking, **T**erpine, **P**henobarbitone, **C**arbamazepine, **R**ifampicin). * **Mnemonic for Enzyme Inhibitors:** "**VITAMIN K**" (**V**alproate, **I**soniazid, **T**imetidine/Cimetidine, **A**miodarone, **M**acrolides, **I**traconazole, **N**euroleptics, **K**etoconazole). * **Lamotrigine Interaction:** Valproate inhibits the glucuronidation of Lamotrigine, doubling its half-life and increasing the risk of life-threatening **Stevens-Johnson Syndrome (SJS)**.

Question 564: What is the drug of choice for a patient with a combination of primary generalized tonic-clonic seizures and absence seizures?

A. Ethosuximide
B. Carbamazepine
C. Valproic acid (Correct Answer)
D. Phenytoin sodium

Explanation: **Explanation:** The correct answer is **Valproic acid**. This is based on the clinical principle of managing **mixed seizure disorders** or generalized epilepsy syndromes. **1. Why Valproic Acid is Correct:** Valproic acid is a broad-spectrum antiepileptic drug (AED). Its mechanism of action is multi-modal: it inhibits voltage-gated sodium channels, increases GABA levels (by inhibiting GABA transaminase), and uniquely blocks **T-type Calcium channels** in the thalamus. While Ethosuximide also blocks T-type channels, Valproic acid’s additional mechanisms make it effective against both Absence seizures (thalamic origin) and Generalized Tonic-Clonic Seizures (GTCS). It is the drug of choice for "Idiopathic Generalized Epilepsy" where multiple seizure types coexist. **2. Why Other Options are Incorrect:** * **Ethosuximide:** It is the drug of choice for *isolated* absence seizures. However, it has a narrow spectrum and is **ineffective** against GTCS. * **Carbamazepine & Phenytoin:** These are narrow-spectrum AEDs primarily used for focal seizures and GTCS. Crucially, they can **exacerbate/worsen** absence and myoclonic seizures. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC) Summary:** * Absence Seizures (Isolated): Ethosuximide. * GTCS, Myoclonic, or Mixed Seizures: Valproic acid. * Trigeminal Neuralgia: Carbamazepine. * Status Epilepticus (Initial): Lorazepam. * **Teratogenicity:** Valproate is highly teratogenic (Neural Tube Defects); **Levetiracetam** or **Lamotrigine** are preferred in pregnancy. * **Side Effects of Valproate:** Hepatotoxicity (monitor LFTs), Pancreatitis, Alopecia, and Weight gain.

Question 565: All of the following are true regarding levodopa except?

A. Half-life of levodopa is 60 to 90 minutes.
B. Levodopa can cause orthostatic hypotension and dyskinesias.
C. Levodopa cannot cross the blood-brain barrier. (Correct Answer)
D. With continued use, the duration of benefit following an individual dose of levodopa becomes progressively shorter.

Explanation: **Explanation:** **1. Why Option C is the correct (false) statement:** The fundamental principle of treating Parkinson’s disease with Levodopa is that **Levodopa can cross the blood-brain barrier (BBB)** via the large neutral amino acid transporter (LAT). Once inside the CNS, it is decarboxylated to Dopamine [1]. Dopamine itself cannot cross the BBB; therefore, Levodopa acts as a "prodrug" to replenish central dopamine levels. **2. Analysis of other options:** * **Option A:** Levodopa has a very short plasma half-life of approximately **60 to 90 minutes**, which contributes to the need for frequent dosing and the development of motor fluctuations [1]. * **Option B:** Peripheral conversion of Levodopa to dopamine can cause **orthostatic hypotension** (via action on D1 receptors in the vasculature). **Dyskinesias** (involuntary movements) are a common long-term side effect related to fluctuating drug levels and striatal hypersensitivity [2]. * **Option D:** This describes the **"wearing-off" phenomenon** [1]. Initially, the brain can store dopamine, but as the disease progresses and nigrostriatal neurons are lost, the duration of benefit mirrors the drug's short half-life. **High-Yield Clinical Pearls for NEET-PG:** * **Carbidopa/Benserazide:** These are peripheral dopa-decarboxylase inhibitors. They do **not** cross the BBB. They are co-administered with Levodopa to prevent peripheral conversion, thereby increasing the amount of Levodopa reaching the brain and reducing peripheral side effects like nausea/vomiting [1]. * **Vitamin B6 (Pyridoxine):** It is a cofactor for decarboxylase. High doses of B6 can increase peripheral metabolism of Levodopa, reducing its efficacy (unless a decarboxylase inhibitor is used). * **Contraindication:** Levodopa can activate latent melanoma; hence, it is contraindicated in patients with suspicious undiagnosed skin lesions.

Question 566: Which of the following drugs is used worldwide for the maintenance therapy of opioid dependence?

A. Naltrexone
B. Methadone (Correct Answer)
C. Pethidine
D. L-NAME

Explanation: **Explanation:** The management of opioid dependence involves two phases: detoxification and **maintenance therapy**. **Methadone** is a long-acting synthetic **mu-opioid receptor agonist**. Its efficacy in maintenance therapy stems from its long half-life (24–36 hours), which prevents withdrawal symptoms and provides a "blockade effect" by inducing cross-tolerance; this prevents the euphoria associated with illicit heroin use. Because it is administered orally, it breaks the ritual of intravenous drug use, allowing for social rehabilitation. **Analysis of Options:** * **Naltrexone (Option A):** While used in opioid de-addiction, it is an **opioid antagonist**. It is used to prevent relapse in highly motivated patients who are already detoxified. It is not the primary choice for maintenance in active dependence because it can precipitate severe withdrawal if opioids are still in the system. * **Pethidine (Option B):** This is a short-acting opioid agonist. It is never used for maintenance because its metabolite, **normeperidine**, is neurotoxic and can cause seizures with chronic use. * **L-NAME (Option D):** This is a non-selective inhibitor of Nitric Oxide Synthase (NOS) used primarily in research settings to study vascular resistance; it has no role in treating opioid dependence. **High-Yield Clinical Pearls for NEET-PG:** * **Buprenorphine** (a partial mu-agonist) is another first-line agent for maintenance therapy, often preferred due to a lower risk of fatal overdose (ceiling effect). * **Clonidine** (alpha-2 agonist) is used to manage the *autonomic symptoms* of acute opioid withdrawal (tachycardia, hypertension) but is not used for maintenance. * **Naloxone** is the drug of choice for **acute opioid poisoning**.

Question 567: Which of the following is a specific inhibitor of the enzyme alcohol dehydrogenase and is useful in the treatment of methanol poisoning?

A. Disulfiram
B. Ethylene glycol
C. Calcium leucovorin
D. Fomepizole (Correct Answer)

Explanation: **Explanation:** The correct answer is **Fomepizole**. **1. Mechanism of Action (Why Fomepizole is correct):** Methanol toxicity is not caused by the alcohol itself, but by its metabolite, **formic acid**. Methanol is converted into formaldehyde by the enzyme **Alcohol Dehydrogenase (ADH)**, and subsequently into formic acid. Fomepizole is a potent **competitive inhibitor of Alcohol Dehydrogenase**. By inhibiting this enzyme, it prevents the formation of toxic metabolites, allowing the parent methanol to be excreted harmlessly by the kidneys. **2. Analysis of Incorrect Options:** * **Disulfiram:** Inhibits **Aldehyde Dehydrogenase (ALDH)**. It is used as aversion therapy in chronic alcoholism by causing the accumulation of acetaldehyde, leading to a "disulfiram-like reaction." It does not stop the initial conversion of methanol. * **Ethylene glycol:** This is a toxic alcohol itself (found in antifreeze). Like methanol, it is metabolized by ADH into toxic oxalic acid. It is a substrate, not an inhibitor. * **Calcium leucovorin:** This is a form of folic acid. While it is used in methanol poisoning, its role is to enhance the breakdown of formic acid into CO₂ and water; it does not inhibit the ADH enzyme. **3. NEET-PG High-Yield Pearls:** * **Antidote of choice:** Fomepizole is preferred over ethanol because it does not cause CNS depression or hypoglycemia and has predictable kinetics. * **Alternative:** If Fomepizole is unavailable, **Ethanol** can be used as it has a higher affinity for ADH than methanol, effectively "occupying" the enzyme. * **Classic Presentation:** Methanol poisoning typically presents with a high anion gap metabolic acidosis and **visual disturbances** ("feeling like being in a snowstorm") due to retinal damage by formic acid. * **Mnemonic:** **F**omepizole **F**ixes methanol/ethylene glycol poisoning by inhibiting the **F**irst enzyme (ADH).

Question 568: Baclofen is used in the treatment of which condition?

A. Schizophrenia
B. Depression
C. Anxiety
D. Spasticity (Correct Answer)

Explanation: **Explanation:** **Baclofen** is a centrally acting skeletal muscle relaxant. Its primary mechanism of action involves acting as a **GABA-B receptor agonist**. It works by increasing potassium conductance, leading to hyperpolarization of neurons in the spinal cord. This inhibits the release of excitatory neurotransmitters (like glutamate and aspartate), thereby reducing monosynaptic and polysynaptic spinal reflexes. This makes it the drug of choice for managing **Spasticity** associated with conditions like Multiple Sclerosis, spinal cord injuries, and amyotrophic lateral sclerosis (ALS). **Analysis of Incorrect Options:** * **A. Schizophrenia:** This is primarily managed with antipsychotics (Dopamine D2 receptor antagonists) like Haloperidol or Risperidone. Baclofen has no role in treating psychosis. * **B. Depression:** This is treated with drugs that increase monoamine levels (SSRIs, SNRIs, TCAs). Baclofen does not modulate serotonin or norepinephrine in a way that treats clinical depression. * **C. Anxiety:** While GABA-A agonists (Benzodiazepines) are used for acute anxiety, Baclofen (GABA-B agonist) is not a standard treatment for anxiety disorders. **High-Yield Clinical Pearls for NEET-PG:** * **Route of Administration:** Can be given orally or via an **intrathecal pump** for severe spasticity. * **Withdrawal Warning:** Abrupt withdrawal of Baclofen can lead to visual hallucinations, seizures, and rebound spasticity; it must be tapered slowly. * **Side Effects:** Drowsiness, muscle weakness, and fatigue are common. * **Comparison:** Unlike Diazepam (GABA-A), Baclofen causes less sedation at effective muscle-relaxant doses.

Question 569: In which of the following disorders is the administration of barbiturates contraindicated?

A. Anxiety disorders
B. Acute intermittent porphyria (Correct Answer)
C. Kernicterus
D. Refractory status epilepticus

Explanation: **Explanation:** **Why Acute Intermittent Porphyria (AIP) is the Correct Answer:** Barbiturates are absolute contraindications in patients with porphyria. The underlying mechanism involves the **induction of hepatic CYP450 enzymes**. Barbiturates increase the synthesis of **ALA synthetase**, the rate-limiting enzyme in the heme biosynthetic pathway. This leads to an overproduction and accumulation of porphyrin precursors (delta-aminolevulinic acid and porphobilinogen), which precipitates a life-threatening acute porphyric attack characterized by abdominal pain, neurological deficits, and psychiatric symptoms. **Analysis of Incorrect Options:** * **Anxiety disorders:** While Benzodiazepines are preferred due to a higher safety profile, barbiturates (like Phenobarbital) were historically used and are not contraindicated, though they are now rarely first-line. * **Kernicterus:** Barbiturates (specifically Phenobarbital) are actually used in the management of unconjugated hyperbilirubinemia. They induce the enzyme **UDP-glucuronosyltransferase (UGT)**, which helps conjugate bilirubin and prevents it from crossing the blood-brain barrier to cause kernicterus. * **Refractory status epilepticus:** Intravenous barbiturates (e.g., Thiopental or Pentobarbital) are indicated as a third-line treatment for refractory status epilepticus to induce a pharmacological coma and suppress seizure activity. **Clinical Pearls for NEET-PG:** * **Enzyme Induction:** Barbiturates are potent "General Enzyme Inducers," leading to many drug-drug interactions (e.g., reducing the efficacy of Warfarin or Oral Contraceptives). * **Respiratory Depression:** Unlike Benzodiazepines, Barbiturates have no "ceiling effect," making them more likely to cause fatal respiratory depression in overdose. * **Specific Antidote:** There is **no specific pharmacological antagonist** for barbiturate overdose; management is purely supportive (alkalinization of urine for Phenobarbital).

Question 570: Which of the following statements about the mechanism of action of anticonvulsants is false?

A. Ethosuximide – K+ channel opener (Correct Answer)
B. Phenytoin – Na+ channel blocker
C. Diazepam – Facilitates GABA action
D. Gabapentin – Increases GABA release

Explanation: ### Explanation The correct answer is **A (Ethosuximide – K+ channel opener)** because this statement is incorrect. **1. Why Option A is the correct answer (False Statement):** Ethosuximide does not act on potassium channels. Its primary mechanism of action is the **inhibition of T-type Ca²⁺ channels** in thalamic neurons. By reducing these low-threshold calcium currents, it suppresses the rhythmic 3 Hz spike-and-wave discharges characteristic of absence seizures. **2. Analysis of Incorrect Options (True Statements):** * **B. Phenytoin:** Acts by causing **use-dependent blockade of voltage-gated Na⁺ channels**. It prolongs the inactivated state of the channel, preventing high-frequency repetitive firing of action potentials. * **C. Diazepam:** As a Benzodiazepine, it acts as a **GABA-A receptor modulator**. It increases the *frequency* of chloride channel opening in the presence of GABA, leading to hyperpolarization. * **D. Gabapentin:** Although designed as a GABA structural analog, it does not act directly on GABA receptors. It binds to the **α2δ-1 subunit of voltage-gated Ca²⁺ channels**, which indirectly leads to an **increase in GABA release** and a decrease in glutamate release. **3. NEET-PG High-Yield Clinical Pearls:** * **Drug of Choice (DOC):** Ethosuximide is the DOC for **Absence Seizures** (Petit mal). However, if the patient has concomitant Generalized Tonic-Clonic Seizures (GTCS), Valproate is preferred. * **Zero-Order Kinetics:** Phenytoin follows capacity-limited (non-linear) elimination at therapeutic concentrations. * **GABA Mechanism Trick:** Remember **"Ben exerts Frequency, Barbs exert Duration"** (Benzodiazepines increase frequency; Barbiturates increase duration of Cl⁻ channel opening). * **Vigabatrin:** Irreversibly inhibits GABA-transaminase (enzyme that breaks down GABA), leading to increased GABA levels. Watch for visual field defects.

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General Anesthetics

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Local Anesthetics

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Sedative-Hypnotics

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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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