Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 51: A 26-year-old female with a history of abortion in a previous pregnancy due to a fetal neural tube defect presents for an antenatal visit. What is the recommended daily dosage of folic acid to be prescribed?

A. 4 micrograms per day
B. 40 micrograms per day
C. 400 micrograms per day
D. 4000 micrograms per day (Correct Answer)

Explanation: ### Explanation The correct answer is **D. 4000 micrograms per day.** **Medical Concept:** Folic acid (Vitamin B9) is essential for DNA synthesis and amino acid metabolism. Its deficiency during early pregnancy is a major risk factor for **Neural Tube Defects (NTDs)** like spina bifida and anencephaly. The dosage of folic acid supplementation depends on the patient's risk profile: * **Low Risk:** Women with no prior history of NTDs should take **400 mcg (0.4 mg)** daily. * **High Risk:** Women with a **previous history of a child/fetus with an NTD** (as in this case) require a "pharmacological dose" of **4000 mcg (4 mg)** daily. This high dose should ideally start 1–3 months before conception and continue through the first trimester. **Analysis of Incorrect Options:** * **A & B (4 mcg & 40 mcg):** These doses are sub-therapeutic and insufficient to prevent NTDs or meet the physiological demands of pregnancy. * **C (400 mcg):** This is the standard prophylactic dose for the **general population** (low-risk). It is insufficient for a patient with a prior history of NTD, where the recurrence risk is significantly higher. **High-Yield Clinical Pearls for NEET-PG:** * **Timing:** Folic acid must be started **pre-conceptionally** because the neural tube closes by the **28th day** of gestation, often before a woman realizes she is pregnant. * **Other High-Risk Groups:** Patients on **anti-epileptic drugs** (e.g., Valproate, Carbamazepine) or those with **Diabetes Mellitus** also require higher doses (up to 5 mg/day) due to the increased risk of malformations. * **Mechanism:** Folic acid prevents NTDs by ensuring proper methylation and reducing homocysteine levels. * **Conversion:** Remember that **4000 mcg = 4 mg**. Examiners often switch units to confuse students.

Question 52: What is the mechanism of action of benzodiazepines?

A. GABA_A - BZD - Cl- channel stimulator (Correct Answer)
B. GABA_B - BZD - Cl- channel stimulator
C. BZD receptor stimulator
D. NMDA receptor stimulator

Explanation: **Explanation:** **Mechanism of Action:** Benzodiazepines (BZDs) are **positive allosteric modulators** of the **GABA_A receptor** complex. They bind to a specific site (BZD receptor) located between the $\alpha$ and $\gamma$ subunits of the GABA_A receptor. This binding increases the **frequency** of chloride ($Cl^-$) channel opening in response to GABA. The resulting influx of chloride ions causes neuronal hyperpolarization, leading to an inhibitory effect on the Central Nervous System (CNS). **Analysis of Options:** * **Option A (Correct):** Accurately describes the target (GABA_A), the drug class (BZD), and the physiological result (opening of the $Cl^-$ channel). * **Option B (Incorrect):** GABA_B receptors are G-protein coupled receptors (GPCRs) linked to $K^+$ channels, not $Cl^-$ channels. **Baclofen** is the prototype agonist for GABA_B. * **Option C (Incorrect):** While BZDs do stimulate the BZD receptor, this option is incomplete as it fails to mention the essential coupling with the GABA_A-$Cl^-$ channel complex. * **Option D (Incorrect):** NMDA receptors are excitatory glutamate receptors. Stimulating them would cause CNS excitation, the opposite of the effect of BZDs. **High-Yield Clinical Pearls for NEET-PG:** * **Frequency vs. Duration:** BZDs increase the **frequency** of channel opening, whereas Barbiturates increase the **duration** of channel opening. * **Antidote:** **Flumazenil** is a specific BZD receptor antagonist used to reverse BZD overdose. * **Site of Action:** BZDs require the presence of GABA to work (GABA-facilitatory), making them safer in overdose compared to barbiturates, which can open channels directly at high doses (GABA-mimetic). * **Metabolism:** Most BZDs undergo hepatic oxidation; however, **L**orazepam, **O**xazepam, and **T**emazepam (**LOT**) undergo direct conjugation, making them safer in elderly patients or those with liver failure.

Question 53: Donepezil has which of the following mechanisms of action?

A. Dopaminergic blockade
B. Cholinesterase inhibition (Correct Answer)
C. Serotonin blockade
D. Histaminergic action

Explanation: **Explanation:** **Mechanism of Action:** Donepezil is a **reversible, non-competitive, and highly selective centrally acting acetylcholinesterase (AChE) inhibitor**. In Alzheimer’s disease, there is a significant deficiency of acetylcholine in the cerebral cortex and hippocampus. Donepezil inhibits the enzyme responsible for breaking down acetylcholine, thereby increasing its concentration at the synaptic cleft and improving cholinergic neurotransmission. This helps in symptomatic improvement of cognitive functions. **Analysis of Incorrect Options:** * **A. Dopaminergic blockade:** This is the mechanism of typical antipsychotics (e.g., Haloperidol). Blocking dopamine in the CNS is used to treat schizophrenia, not cognitive decline. * **C. Serotonin blockade:** Drugs like Ondansetron (5-HT3 antagonist) or certain atypical antipsychotics block serotonin receptors. This is unrelated to the primary pathology of Alzheimer's. * **D. Histaminergic action:** Histamine agonists are not standard treatments for CNS neurodegeneration. Conversely, H1 antagonists (like diphenhydramine) are often avoided in the elderly due to their anticholinergic side effects which worsen dementia. **NEET-PG High-Yield Pearls:** * **Indications:** Donepezil is the first-line treatment for mild, moderate, and severe Alzheimer’s disease. * **Selectivity:** Unlike Tacrine, Donepezil is selective for AChE over butyrylcholinesterase, leading to fewer peripheral side effects and **no hepatotoxicity**. * **Pharmacokinetics:** It has a long half-life (~70 hours), allowing for **once-daily dosing**, usually at bedtime to minimize GI side effects. * **Side Effects:** Primarily cholinergic (SLUDGE): Diarrhea, nausea, and **bradycardia** (important to monitor in patients with heart block). * **Other Drugs in Class:** Rivastigmine (pseudo-irreversible) and Galantamine.

Question 54: What is the prophylactic plasma concentration range of lithium in mEq, and which of the following values does NOT fall within it?

A. 0.5 (Correct Answer)
B. 0.8
C. 0.6
D. 1.0

Explanation: Lithium is the gold standard for the treatment and prophylaxis of Bipolar Affective Disorder (BPAD). Because it has a **narrow therapeutic index** [2], monitoring serum lithium levels is critical to ensure efficacy and avoid toxicity. The therapeutic ranges for lithium are categorized based on the phase of treatment: 1. **Acute Mania:** 0.8 to 1.2 mEq/L (sometimes up to 1.5 mEq/L). 2. **Prophylaxis (Maintenance):** **0.6 to 1.2 mEq/L** [1]. **Why Option A is the correct answer:** The prophylactic range is strictly defined as **0.6–1.2 mEq/L** [1]. A value of **0.5 mEq/L** falls below this threshold. While it may provide some mood stabilization in sensitive patients, it is generally considered sub-therapeutic for effective long-term prophylaxis against manic or depressive relapses. **Analysis of Incorrect Options:** * **B (0.8 mEq/L):** This is the ideal "sweet spot" for maintenance therapy, balancing efficacy with a lower risk of side effects. * **C (0.6 mEq/L):** This is the lower limit of the prophylactic range. * **D (1.0 mEq/L):** This is well within the prophylactic range (0.6–1.2 mEq/L) and is often targeted for patients who do not respond to lower doses. **NEET-PG High-Yield Pearls:** * **Toxicity Levels:** Mild toxicity starts at >1.5 mEq/L; severe toxicity (seizures, coma) occurs at >2.0 mEq/L [2]. * **Monitoring:** Samples should be drawn **12 hours after the last dose** (trough levels). * **Drug Interactions:** Thiazides, NSAIDs, and ACE inhibitors increase lithium levels (by decreasing renal clearance), while Caffeine and Theophylline decrease lithium levels [1]. * **Side Effects:** Ebstein’s anomaly (teratogenic), Nephrogenic Diabetes Insipidus, and Hypothyroidism [1].

Question 55: Which drug is used in absence seizures and has a narrow spectrum of antiepileptic activity?

A. Lamotrigine
B. Ethosuximide (Correct Answer)
C. Sodium valproate
D. Primidone

Explanation: **Explanation** **Ethosuximide** is the drug of choice for **absence seizures (petit mal)** in children. Its mechanism of action involves the selective inhibition of **T-type Ca²⁺ channels** in thalamic neurons, which are responsible for the characteristic 3-Hz spike-and-wave discharges seen on EEG in absence epilepsy. It is considered a **narrow-spectrum** agent because it is ineffective against generalized tonic-clonic seizures (GTCS) or focal seizures. **Analysis of Incorrect Options:** * **Sodium Valproate:** While it is also a drug of choice for absence seizures, it is a **broad-spectrum** antiepileptic. It is preferred over ethosuximide only if the patient has concomitant GTCS. * **Lamotrigine:** This is a broad-spectrum agent used for various seizures, including absence seizures (as an alternative). However, it is not the primary narrow-spectrum drug for this indication. * **Primidone:** A barbiturate derivative primarily used for focal and generalized tonic-clonic seizures. It is not used for absence seizures and can sometimes exacerbate them. **High-Yield NEET-PG Pearls:** * **Drug of Choice (DOC):** Ethosuximide is the DOC for pure absence seizures. If absence seizures coexist with GTCS, **Valproate** becomes the DOC. * **Side Effects of Ethosuximide:** Remember the mnemonic **EFGHIJ** – **E**thosuximide causes **F**atigue, **G**I distress, **H**eadache, **I**tching (Urticaria), and Stevens-**J**ohnson syndrome. * **EEG Finding:** Absence seizures are classically associated with **3-Hz spike-and-wave** patterns. * **Contraindication:** Avoid **Carbamazepine** and **Phenytoin** in absence seizures as they may worsen the condition.

Question 56: Which of the following agents does not act via GABAA-Cl– channel complex receptors?

A. Zopiclone
B. Benzodiazepines
C. Thiopentone
D. Promethazine (Correct Answer)

Explanation: **Explanation:** The **GABA-A receptor** is a ligand-gated ion channel (ionotropic receptor) that, when activated, increases chloride conductance, leading to neuronal hyperpolarization and CNS depression. **Why Promethazine is the Correct Answer:** Promethazine is a **first-generation H1-antihistamine** with significant anticholinergic and alpha-blocking properties. It does not interact with the GABA-A receptor complex. Its sedative effects are primarily due to the blockade of central H1 receptors and muscarinic receptors in the brain. **Analysis of Incorrect Options:** * **Benzodiazepines (B):** These are positive allosteric modulators that bind to a specific site on the GABA-A receptor, increasing the **frequency** of chloride channel opening. * **Thiopentone (C):** As a barbiturate, it binds to a distinct site on the GABA-A receptor to increase the **duration** of chloride channel opening. At high doses, it can also act as a GABA-mimetic. * **Zopiclone (A):** This is a "Z-drug" (non-benzodiazepine hypnotic). Despite having a different chemical structure than benzodiazepines, it binds specifically to the **alpha-1 subunit** of the GABA-A receptor to exert its hypnotic effect. **NEET-PG High-Yield Pearls:** * **GABA-A vs. GABA-B:** GABA-A is ionotropic (Chloride channel); GABA-B is metabotropic (G-protein coupled, linked to K+ channels). **Baclofen** is a GABA-B agonist. * **Flumazenil:** A competitive antagonist that reverses the effects of Benzodiazepines and Z-drugs, but **not** Barbiturates. * **Promethazine Clinical Use:** Often used for motion sickness and as a pre-anesthetic medication due to its antiemetic and sedative properties.

Question 57: Which antiepileptic drug is not associated with enzyme induction or inhibition?

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

Explanation: ### Explanation The correct answer is **Ethosuximide (Option D)**. **1. Why Ethosuximide is Correct:** Ethosuximide is the drug of choice for **Absence Seizures**. Unlike many traditional antiepileptics, it is metabolized by the liver but does not significantly induce or inhibit the Cytochrome P450 (CYP) enzyme system. This makes it relatively "inert" regarding metabolic drug-drug interactions, a key characteristic that distinguishes it from the other options provided. **2. Why the Other Options are Incorrect:** * **Phenytoin (Option A):** A potent **enzyme inducer** (specifically CYP3A4 and CYP2C9). It also exhibits zero-order kinetics at high therapeutic doses, making its metabolism easily saturable. * **Valproate (Option B):** A notorious **enzyme inhibitor**. It inhibits the metabolism of other drugs like Phenobarbital and Lamotrigine, leading to increased toxicity. * **Carbamazepine (Option C):** A powerful **enzyme inducer**. It is unique because it is an **auto-inducer**, meaning it induces its own metabolism over the first few weeks of therapy. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mnemonic for Enzyme Inducers:** "GP Cell Phones" (Griseofulvin, Phenytoin, Carbamazepine, Rifampin, Phenobarbital). * **Mnemonic for Enzyme Inhibitors:** "VITAMIN K" (Valproate, Isoniazid, Timetidine/Cimetidine, Amiodarone, Macrolides, Itraconazole, Ketoconazole). * **Newer Antiepileptics:** Drugs like **Levetiracetam** and **Gabapentin** are also notable for having minimal to no effect on hepatic enzymes, making them safer in polypharmacy. * **Ethosuximide Mechanism:** It works by inhibiting **T-type Calcium channels** in thalamic neurons. Its most common side effects are GI distress and drowsiness (Mnemonic: **EFGHI** – **E**thosuximide causes **F**atigue, **G**I distress, **H**eadache, **I**tching/Urticaria).

Question 58: Which of the following opioids should not be used with MAO inhibitors?

A. Morphine
B. Pentazocine
C. Buprenorphine
D. Pethidine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Pethidine** (Meperidine). The interaction between Pethidine and Monoamine Oxidase Inhibitors (MAOIs) is a classic, high-yield pharmacological contraindication due to the risk of two distinct types of toxic reactions: 1. **Excitatory Reaction (Serotonin Syndrome):** Pethidine acts as a weak serotonin reuptake inhibitor. When combined with MAOIs (which prevent serotonin breakdown), it leads to an excessive accumulation of serotonin. This results in "Serotonin Syndrome," characterized by hyperpyrexia (severe fever), agitation, muscle rigidity, seizures, and cardiovascular instability. 2. **Depressive Reaction:** MAOIs can inhibit the hepatic microsomal enzymes that metabolize Pethidine, leading to toxic levels of the drug, resulting in respiratory depression, hypotension, and coma. **Analysis of Incorrect Options:** * **A. Morphine:** Unlike Pethidine, Morphine does not significantly affect serotonin reuptake. While caution is always advised when mixing CNS depressants, it does not trigger the life-threatening excitatory reaction seen with Pethidine. * **B. Pentazocine & C. Buprenorphine:** These are agonist-antagonist/partial agonist opioids. While they have complex side effect profiles, they do not possess the specific serotonergic activity required to precipitate Serotonin Syndrome when used with MAOIs. **High-Yield Clinical Pearls for NEET-PG:** * **The "Pethidine Rule":** Always remember the triad of Pethidine + MAOI = Hyperpyrexia, Delirium, and Convulsions. * **Other Serotonergic Opioids:** Tramadol, Fentanyl, and Methadone also carry a risk of Serotonin Syndrome when combined with MAOIs or SSRIs. * **Metabolite Fact:** Pethidine is metabolized to **norpethidine**, which is neurotoxic and can cause seizures, especially in patients with renal failure.

Question 59: What is the drug of choice for status epilepticus?

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

Explanation: **Explanation:** **Status Epilepticus (SE)** is a medical emergency defined as a continuous seizure lasting more than 5 minutes or recurrent seizures without recovery of consciousness between episodes. **Why Lorazepam is the Correct Answer:** Benzodiazepines (BZDs) are the first-line agents for terminating acute seizures because they enhance GABA-A mediated inhibition. **Lorazepam (IV)** is the preferred drug of choice (DOC) over Diazepam because it is less lipid-soluble. This allows it to remain in the brain for a longer duration (high redistribution half-life), providing a more sustained anticonvulsant effect (up to 12–24 hours) compared to Diazepam, which redistributes rapidly into fat stores, leading to a shorter duration of action. **Analysis of Incorrect Options:** * **Valproate (A):** While IV Valproate is an effective second-line agent for SE (especially if BZDs fail), it is not the initial drug of choice. * **Phenytoin (B):** Phenytoin (or Fosphenytoin) is used as a second-line agent to prevent the *recurrence* of seizures after the acute episode has been terminated by a BZD. It has a slow onset of action and cannot be used for immediate termination. * **Carbamazepine (D):** This is the DOC for Focal (Partial) seizures and Trigeminal Neuralgia. It is not used in SE and can actually worsen certain generalized seizures (like absence or myoclonic seizures). **High-Yield Clinical Pearls for NEET-PG:** * **Order of Management:** 1st line: Lorazepam (IV); 2nd line: Fosphenytoin, Valproate, or Levetiracetam; 3rd line (Refractory SE): Midazolam infusion, Propofol, or Thiopental. * **Route:** If IV access is unavailable, **Intramuscular (IM) Midazolam** is the preferred alternative. * **Fosphenytoin vs. Phenytoin:** Fosphenytoin is a prodrug; it is preferred over Phenytoin because it is water-soluble, causes less local irritation (Purple Glove Syndrome), and can be infused faster.

Question 60: Which of the following is a respiratory analeptic drug?

A. Picrotoxin
B. Methylphenidate
C. Caffeine
D. Doxapram (Correct Answer)

Explanation: ### Explanation **Correct Answer: D (Doxapram)** **Why Doxapram is the Correct Answer:** Doxapram is a potent **respiratory analeptic** (respiratory stimulant). It acts by stimulating the peripheral chemoreceptors in the carotid bodies, which in turn increases the tidal volume and respiratory rate. At higher doses, it directly stimulates the respiratory center in the medulla. It is primarily used in clinical settings to treat acute respiratory failure or to hasten recovery from general anesthesia-induced respiratory depression. **Analysis of Incorrect Options:** * **A. Picrotoxin:** This is a potent **GABA-A receptor antagonist**. While it is a CNS stimulant, it is highly toxic and primarily used as a laboratory tool to induce convulsions in animal models. It is not used clinically as a respiratory stimulant due to its narrow therapeutic index and high risk of seizures. * **B. Methylphenidate:** This is a **central sympathomimetic** used primarily in the treatment of ADHD and Narcolepsy. It increases synaptic dopamine and norepinephrine but does not have a significant direct effect on the respiratory drive. * **C. Caffeine:** While caffeine (a methylxanthine) does stimulate the respiratory center and is used for **Apnea of Prematurity**, it is classified more broadly as a CNS stimulant/adenosine antagonist rather than a primary respiratory analeptic like Doxapram. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Doxapram:** Stimulates carotid chemoreceptors (low dose) → Medullary respiratory center (high dose). * **Drug of Choice for Apnea of Prematurity:** Caffeine citrate (preferred over Theophylline due to a better safety profile and longer half-life). * **Analeptic Warning:** Most analeptics have a narrow therapeutic window; the dose required to stimulate respiration is often close to the dose that induces generalized seizures. * **Contraindication:** Doxapram should be avoided in patients with mechanical disorders of ventilation (e.g., severe obstruction) or cardiovascular disease.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free