Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 631: Which among the following drugs is contraindicated in renal failure?

A. Pethidine (Correct Answer)
B. Morphine
C. Fentanyl
D. Atracurium

Explanation: **Explanation:** **Pethidine (Meperidine)** is the correct answer because of its metabolic profile. It is metabolized in the liver to **norpethidine**, a toxic metabolite. Norpethidine is primarily excreted by the kidneys. In patients with renal failure, norpethidine accumulates, leading to CNS hyper-excitability. This manifests as tremors, myoclonus, and potentially fatal **seizures**. Unlike opioid overdose, these seizures are not reversed by Naloxone; in fact, Naloxone may worsen them by lowering the seizure threshold. **Analysis of Incorrect Options:** * **Morphine:** While its metabolite (Morphine-6-glucuronide) can accumulate in renal failure and cause prolonged respiratory depression, it is not strictly contraindicated; it can be used with extreme caution and dose adjustment. Pethidine is more dangerous due to the seizure risk. * **Fentanyl:** This is considered the **opioid of choice** in renal failure. It has no active metabolites and is primarily cleared by hepatic metabolism, making it safe for patients with impaired kidney function. * **Atracurium:** This neuromuscular blocker undergoes **Hofmann elimination** (spontaneous non-enzymatic degradation in the plasma). Its clearance is independent of renal or hepatic function, making it safe in renal failure. **High-Yield Clinical Pearls for NEET-PG:** * **Pethidine + MAO Inhibitors:** Can cause "Serotonin Syndrome" (hyperpyrexia, coma). * **Drug of choice in Renal Failure:** Fentanyl (Opioid), Atracurium/Cisatracurium (Muscle relaxant). * **Pethidine** is preferred over Morphine in biliary colic because it causes less spasm of the Sphincter of Oddi.

Question 632: Which of the following is an antidepressant drug?

A. Pimozide
B. Haloperidol
C. Thioridazine
D. Citalopram (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Citalopram** [1]. **1. Why Citalopram is correct:** Citalopram is a potent and highly selective **Selective Serotonin Reuptake Inhibitor (SSRI)** [1]. It works by inhibiting the reuptake of serotonin (5-HT) at the presynaptic terminal, thereby increasing the concentration of serotonin in the synaptic cleft [1]. SSRIs are currently the first-line treatment for Major Depressive Disorder (MDD) due to their favorable side-effect profile compared to older antidepressants like TCAs or MAOIs [1], [2]. **2. Why the other options are incorrect:** * **Pimozide (A):** This is a high-potency **typical antipsychotic** (diphenylbutylpiperidine derivative). It is primarily used in the management of Tourette’s syndrome and resistant schizophrenia. * **Haloperidol (B):** A prototypical high-potency **typical antipsychotic** (butyrophenone). It acts as a potent Dopamine D2 receptor antagonist and is used for schizophrenia and acute psychosis. * **Thioridazine (C):** A low-potency **typical antipsychotic** (phenothiazine). It is rarely used today due to its significant side effects, including pigmentary retinopathy and QTc prolongation. **3. NEET-PG High-Yield Clinical Pearls:** * **Escitalopram:** The *S-enantiomer* of citalopram [1], [3]; it is considered the most selective SSRI [3] and is more potent than the racemic mixture [2]. * **Side Effects of SSRIs:** Most common include GI upset (nausea), sexual dysfunction (delayed ejaculation), and insomnia [3]. * **Serotonin Syndrome:** A life-threatening condition (hyperthermia, rigidity, myoclonus) that can occur if SSRIs are combined with MAOIs. * **Thioridazine Warning:** It is the antipsychotic most commonly associated with **Torsades de Pointes** (QT prolongation) and **retinitis pigmentosa** (at high doses).

Question 633: Compared to other antidepressant drugs, mirtazapine has the distinct ability to act as an antagonist of which receptor?

A. Beta receptors
B. D2 receptors
C. Alpha 2 receptors (Correct Answer)
D. 5-HT receptors

Explanation: **Explanation:** Mirtazapine is classified as a **NaSSA (Noradrenergic and Specific Serotonergic Antidepressant)**. Its primary mechanism of action is the **antagonism of central presynaptic alpha-2 ($\alpha_2$) autoreceptors and heteroreceptors**. By blocking these receptors, mirtazapine removes the "negative feedback" brake on neurotransmitter release. This results in an increased release of both **Norepinephrine (NE)** and **Serotonin (5-HT)** into the synaptic cleft. This unique mechanism distinguishes it from SSRIs or TCAs, which primarily work by inhibiting reuptake. **Analysis of Options:** * **Option A (Beta receptors):** Mirtazapine does not have significant activity at beta-adrenergic receptors. Beta-blockers are used for anxiety symptoms but are not antidepressants. * **Option B (D2 receptors):** Antagonism of D2 receptors is the hallmark of antipsychotics (e.g., Haloperidol). Mirtazapine lacks significant dopaminergic activity. * **Option D (5-HT receptors):** While mirtazapine *does* antagonize 5-HT2 and 5-HT3 receptors (which reduces side effects like anxiety and nausea), its **distinctive** therapeutic mechanism for increasing monoamine levels is its $\alpha_2$ antagonism. **NEET-PG High-Yield Pearls:** 1. **Weight Gain & Sedation:** Mirtazapine is a potent **H1 receptor antagonist**, leading to significant sedation and weight gain (useful in elderly patients with insomnia and cachexia). 2. **Sexual Dysfunction:** Unlike SSRIs, mirtazapine has a **minimal risk of sexual dysfunction** because it blocks 5-HT2 receptors. 3. **Antiemetic Effect:** Due to **5-HT3 antagonism**, it is less likely to cause the GI distress/nausea common with SSRIs.

Question 634: Rivastigmine is indicated for which of the following conditions?

A. Depression
B. Alzheimer's disease (Correct Answer)
C. Schizophrenia
D. Obsessive-compulsive disorder

Explanation: **Explanation:** **Rivastigmine** is a reversible, non-competitive **Cholinesterase Inhibitor (ChEI)**. It works by inhibiting both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes in the synaptic cleft. **Why Alzheimer’s Disease is Correct:** The "Cholinergic Hypothesis" of Alzheimer’s disease (AD) suggests that deficiency in acetylcholine (ACh) leads to cognitive decline. Rivastigmine increases the concentration of ACh in the brain, thereby improving memory, cognition, and daily functioning. It is unique because it is available as a **transdermal patch**, which reduces gastrointestinal side effects and improves patient compliance. **Why Other Options are Incorrect:** * **Depression:** Managed primarily with SSRIs, SNRIs, or TCAs which modulate serotonin and norepinephrine, not acetylcholine. * **Schizophrenia:** Primarily involves dopamine dysregulation; treated with antipsychotics (D2 receptor antagonists). * **Obsessive-Compulsive Disorder (OCD):** Treated with high-dose SSRIs or Clomipramine (a TCA). **High-Yield Clinical Pearls for NEET-PG:** * **Indications:** Rivastigmine is FDA-approved for mild-to-moderate **Alzheimer’s disease** and **Parkinson’s disease dementia**. * **Mechanism:** It is a "pseudo-irreversible" inhibitor because it dissociates very slowly from the enzyme. * **Side Effects:** Primarily cholinergic (SLUDGE)—nausea, vomiting, diarrhea, and bradycardia. * **Other ChEIs for AD:** Donepezil (long-acting) and Galantamine. * **NMDA Antagonist:** Memantine is often added to ChEIs for moderate-to-severe Alzheimer’s.

Question 635: Neural tube defect is an adverse effect of which medication?

A. Valproate (Correct Answer)
B. Phenytoin
C. Diazoxide
D. None of the above

Explanation: **Explanation:** **Valproate (Sodium Valproate)** is the correct answer because it is the antiepileptic drug most strongly associated with **Neural Tube Defects (NTDs)**, specifically **spina bifida**. The risk is approximately 1–2%, which is significantly higher than the general population. The underlying mechanism involves the inhibition of **histone deacetylase (HDAC)** and interference with **folate metabolism**, which are critical for proper neural tube closure during the first trimester. **Analysis of Incorrect Options:** * **Phenytoin:** While teratogenic, it is primarily associated with **Fetal Hydantoin Syndrome**, characterized by craniofacial anomalies (cleft lip/palate), hypoplastic phalanges, and fingernails. * **Diazoxide:** This is a K+ channel opener used for hypertensive emergencies or insulinomas. It is not a standard antiepileptic and is not typically associated with neural tube defects. **High-Yield Clinical Pearls for NEET-PG:** 1. **Fetal Valproate Syndrome:** Beyond NTDs, it includes atrial septal defects, cleft palate, and developmental delay. 2. **Prevention:** To reduce the risk of NTDs in women of childbearing age taking valproate, high-dose **Folic Acid (4–5 mg/day)** supplementation is mandatory starting before conception. 3. **Safest in Pregnancy:** **Lamotrigine** and **Levetiracetam** are currently considered the safest antiepileptic options regarding major malformations. 4. **Other Valproate Side Effects:** Remember the mnemonic **VALPROATE**: **V**omiting, **A**lopecia, **L**iver toxicity (Hepatotoxicity), **P**ancreatitis, **R**etained fat (Weight gain), **O**edema, **A**norexia, **T**remors/Teratogenicity, and **E**nzyme inhibitor (unlike most AEDs which are inducers).

Question 636: L-Dopa is combined with carbidopa in the treatment of parkinsonism to:

A. Decrease the peripheral metabolism of levodopa
B. Decrease the peripheral metabolism of levodopa (Correct Answer)
C. Increase the dose of levodopa required
D. Inhibit conversion of levodopa to dopamine in the CNS

Explanation: ### Explanation **1. Why Option B is Correct:** Levodopa (L-Dopa) is a precursor to dopamine that can cross the blood-brain barrier (BBB). However, when administered alone, more than 95% of L-Dopa is decarboxylated into dopamine in the peripheral tissues by the enzyme **DOPA decarboxylase**. Since dopamine cannot cross the BBB, this leads to systemic side effects (nausea, vomiting, cardiac arrhythmias) and reduced therapeutic efficacy. **Carbidopa** is a peripheral DOPA decarboxylase inhibitor that does not cross the BBB. By inhibiting the peripheral metabolism of L-Dopa, carbidopa ensures that a higher concentration of L-Dopa reaches the brain, where it can be converted into dopamine. **2. Why Other Options are Incorrect:** * **Option C:** Combining carbidopa actually **decreases** the required dose of levodopa by approximately 75–80%. * **Option D:** Carbidopa is specifically designed **not** to cross the BBB. If it inhibited the conversion of L-Dopa to dopamine in the CNS, the drug would be therapeutically useless, as dopamine is the required neurotransmitter in the striatum. **3. High-Yield NEET-PG Clinical Pearls:** * **The Ratio:** L-Dopa and Carbidopa are usually administered in a **4:1 or 10:1 ratio** (e.g., 100mg L-Dopa + 25mg Carbidopa). * **Side Effects:** While carbidopa reduces peripheral side effects (nausea/vomiting), it may actually **increase or worsen central side effects** like dyskinesias and hallucinations because more dopamine is being produced in the brain. * **Vitamin Interaction:** Pyridoxine (Vitamin B6) is a cofactor for DOPA decarboxylase. Taking B6 alone increases peripheral metabolism of L-Dopa, but this interaction is **abolished** when L-Dopa is combined with carbidopa. * **The "On-Off" Phenomenon:** This refers to the fluctuations in motor performance seen with long-term L-Dopa therapy.

Question 637: What is the drug of choice for absence seizures?

A. Carbamazepine
B. Phenytoin
C. ACTH
D. Ethosuximide (Correct Answer)

Explanation: **Explanation:** **Ethosuximide** is the drug of choice for **absence seizures** (petit mal) because it specifically inhibits **T-type Ca²⁺ channels** in thalamic neurons. These channels are responsible for the characteristic 3 Hz spike-and-wave discharges seen on EEG during absence episodes. Ethosuximide is preferred due to its high efficacy and relatively narrow side-effect profile compared to alternatives. **Analysis of Incorrect Options:** * **Carbamazepine (A) & Phenytoin (B):** These are sodium channel blockers used for focal and generalized tonic-clonic seizures. Crucially, they are **contraindicated** in absence seizures as they can paradoxically aggravate or worsen the condition. * **ACTH (C):** This is the drug of choice for **Infantile Spasms** (West Syndrome), not absence seizures. **High-Yield Clinical Pearls for NEET-PG:** * **Valproate vs. Ethosuximide:** While Ethosuximide is the drug of choice for *pure* absence seizures, **Sodium Valproate** is the drug of choice if absence seizures are associated with generalized tonic-clonic seizures (GTCS), as it is a broad-spectrum antiepileptic. * **EEG Hallmark:** Absence seizures are characterized by a **3 Hz spike-and-wave pattern**. * **Side Effects of Ethosuximide:** Remember the mnemonic **EFGHIJ** – **E**thosuximide causes **F**atigue, **G**I distress, **H**eadache, **I**tching (Urticaria), and **J**oviality (Euphoria). * **Drug of Choice Summary:** * Trigeminal Neuralgia: Carbamazepine * Status Epilepticus: Lorazepam (IV) * Myoclonic Seizures: Sodium Valproate

Question 638: Lowest risk of malformations is seen with which of the following antiepileptic drugs?

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

Explanation: **Explanation:** The risk of congenital malformations (teratogenicity) is a critical consideration when treating epilepsy in women of childbearing age. Among the options provided, **Lamotrigine** is associated with the lowest risk of major congenital malformations (MCMs). **1. Why Lamotrigine is Correct:** Large pregnancy registries (such as EURAP) consistently show that Lamotrigine (along with Levetiracetam) has a low malformation rate, typically cited around **2–3%**, which is comparable to the baseline risk in the general population. It is considered one of the safest first-line options for managing epilepsy during pregnancy. **2. Why the Other Options are Incorrect:** * **Valproate (Option B):** This is the **most teratogenic** antiepileptic drug (risk ~10.7%). It is specifically associated with **Neural Tube Defects (NTDs)** like spina bifida, as well as cognitive impairment and "Fetal Valproate Syndrome." * **Phenobarbitone (Option A):** Associated with a higher risk (~5–6%) of cardiac defects and orofacial clefts. * **Carbamazepine (Option D):** Carries a moderate risk (~4–5%), primarily associated with neural tube defects and craniofacial abnormalities, though the risk is lower than Valproate. **Clinical Pearls for NEET-PG:** * **Highest Risk:** Valproate (Avoid in pregnancy unless no alternative exists). * **Safest Drugs:** Lamotrigine and Levetiracetam. * **Specific Defect:** Phenytoin is classically associated with **Fetal Hydantoin Syndrome** (hypoplastic phalanges, cleft lip/palate). * **Management:** All women on AEDs should receive high-dose **Folic acid (5 mg/day)** pre-conceptionally to reduce the risk of NTDs. * **Vitamin K:** Enzyme-inducing AEDs (Phenobarbital, Phenytoin, Carbamazepine) can cause neonatal hemorrhage; Vitamin K prophylaxis is essential at birth.

Question 639: What is the duration of action of flumazenil?

A. 5 minutes
B. 10 minutes
C. 20 minutes
D. 1-2 hours (Correct Answer)

Explanation: **Explanation:** **1. Why the correct answer is right:** Flumazenil is a competitive antagonist at the benzodiazepine (BZD) binding site on the $GABA_A$ receptor. It is used to reverse the sedative effects of benzodiazepines. Its pharmacokinetics are characterized by rapid hepatic clearance and a short elimination half-life. The clinical duration of action is typically **1-2 hours**. Because most benzodiazepines (like diazepam or midazolam) have a significantly longer half-life than flumazenil, the antagonist effect wears off before the agonist is cleared. **2. Why the incorrect options are wrong:** * **Options A, B, and C (5, 10, 20 minutes):** These durations are too short. While the **onset** of action of flumazenil is very rapid (1-2 minutes), its therapeutic effect persists longer than 20 minutes. These options represent the distribution phase rather than the clinical duration of action. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **"Resedation" Risk:** This is the most critical clinical concept. Since flumazenil’s duration (1-2 hours) is shorter than most BZDs, patients may slip back into respiratory depression or sedation once flumazenil wears off. Repeated doses or a continuous infusion may be required. * **Precipitation of Seizures:** Flumazenil can trigger seizures in patients who are on long-term BZD therapy (due to withdrawal) or in cases of mixed overdose involving tricyclic antidepressants (TCAs). * **Specific Antagonist:** It reverses BZDs and "Z-drugs" (Zolpidem, Zopiclone), but it does **not** reverse the effects of barbiturates, ethanol, or opioids. * **Route:** It is administered intravenously only.

Question 640: What is the drug of choice for relapsing-remitting multiple sclerosis?

A. Alpha interferon
B. Beta interferon (Correct Answer)
C. Gamma interferon
D. Natalizumab

Explanation: **Relapsing-Remitting Multiple Sclerosis (RRMS)** is the most common form of MS, characterized by episodes of new or worsening neurological symptoms followed by periods of partial or complete recovery [1]. **Why Beta Interferon is the Correct Answer:** **Interferon-beta (IFN-β)**, specifically types 1a and 1b, is considered a first-line disease-modifying therapy (DMT) for RRMS. It works by modulating the immune response: it reduces the movement of inflammatory T-cells across the blood-brain barrier and increases the production of anti-inflammatory cytokines. This leads to a decreased frequency of clinical relapses and a reduction in the formation of new brain lesions on MRI [1], [2]. **Analysis of Incorrect Options:** * **Alpha Interferon (IFN-α):** Primarily used in the treatment of viral infections (Hepatitis B and C) and certain malignancies (Hairy cell leukemia, Kaposi sarcoma). It is not indicated for MS [2]. * **Gamma Interferon (IFN-γ):** This is a pro-inflammatory cytokine. Administering IFN-γ actually **exacerbates** MS symptoms and increases relapse rates; it is used clinically for Chronic Granulomatous Disease (CGD) [2]. * **Natalizumab:** While highly effective for RRMS, it is generally reserved as a second-line agent or for rapidly progressing disease due to the risk of **Progressive Multifocal Leukoencephalopathy (PML)** caused by JC virus reactivation. **High-Yield Clinical Pearls for NEET-PG:** * **Glatiramer acetate** is another first-line alternative for RRMS, especially in patients who cannot tolerate interferons [1]. * **Drug of Choice for Acute Attack:** Intravenous Methylprednisolone (High-dose steroids). * **Oral Drugs for MS:** Fingolimod (S1P modulator), Teriflunomide, and Dimethyl fumarate. * **Side Effects of IFN-β:** Flu-like symptoms (most common) and injection site reactions [2]. Pre-medication with NSAIDs is often recommended.

Practice by Chapter

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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