Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 671: Which of the following statements is not true regarding benzodiazepines and barbiturates?

A. Both can be used as a sedative hypnotic
B. With alcohol it causes CNS depression
C. Both act on GABA-A
D. Flumazenil can be used for severe alcohol withdrawal symptoms (Correct Answer)

Explanation: ***Flumazenil can be used for severe alcohol withdrawal symptoms*** - This statement is **not true** because **Flumazenil** is a competitive antagonist at the **GABA-A receptor** that specifically reverses the effects of benzodiazepines. - Flumazenil is **contraindicated** in severe alcohol withdrawal, especially when BZDs are used for treatment, as it can precipitate **seizures**. ***Both act on GABA-A*** - This statement is true; both benzodiazepines and barbiturates are **positive allosteric modulators** of the **GABA-A receptor**. - Benzodiazepines increase the **frequency of chloride ion channel opening**, while barbiturates increase the **duration of opening**. ***With alcohol it causes CNS depression*** - This statement is true; both classes exhibit a **synergistic effect** with alcohol, leading to rapid and profound **CNS depression**. - Combining these drugs dramatically increases the risk of respiratory depression, coma, and lethal overdose due to enhanced inhibitory neurotransmission. ***Both can be used as a sedative hypnotic*** - This statement is true; both benzodiazepines (e.g., Diazepam, Clonazepam) and barbiturates (e.g., Phenobarbital) are classified as central nervous system depressants used to produce **sedation** (calmness) or induce **hypnosis** (sleep). - Barbiturates are largely replaced by safer BZDs for hypnotic use due to their lower therapeutic index and higher dependence potential.

Question 672: Which of the following anti-epileptics is not given during pregnancy?

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

Explanation: ***Correct: Valproate*** - Valproate (Valproic Acid) is the anti-epileptic drug (AED) with the **highest teratogenic risk**, particularly causing **Neural Tube Defects (NTDs)** like spina bifida (1-2% risk), in a dose-dependent manner - It is **strictly contraindicated** during pregnancy (FDA Category X) due to increased risks of **major congenital malformations (10-20%)** and long-term neurodevelopmental consequences, including lower IQ and increased risk of **Autism Spectrum Disorder** in the child - This is the AED that should **NOT be given during pregnancy** *Incorrect: Lamotrigine* - Lamotrigine is generally considered one of the **safer AEDs** used in pregnancy, often a preferred alternative when monotherapy is necessary - While initial data suggested a small risk of oral clefts, current extensive evidence shows it has a **low overall risk** of major congenital malformations - Can be safely used during pregnancy when needed *Incorrect: Levetiracetam* - Levetiracetam (Keppra) is highly favored during pregnancy because it has one of the **lowest risks** of causing major congenital malformations among all AEDs - It is often recommended as the **drug of choice** for women who require AED continuation during gestation due to its favorable safety profile for both physical and neurodevelopmental outcomes *Incorrect: Carbamazepine* - Carbamazepine increases the risk of teratogenicity, classically associated with a risk of **NTDs** (~1%, lower than Valproate) and minor craniofacial defects like cleft lip/palate - Although generally reserved for situations where safer alternatives are ineffective, it **can still be used** when benefits outweigh risks - not absolutely contraindicated like valproate

Question 673: For benzodiazepines and barbiturates, which of the following is true?

A. Flumazenil is the antidote for barbiturate
B. Cause additive sedation with alcohol
C. Thiopentone has short duration of action due to metabolism
D. Both have effect on GABA-A chloride channel (Correct Answer)

Explanation: ***Both have effect on GABA-A chloride channel*** Both benzodiazepines and barbiturates act as **positive allosteric modulators** of the **GABA-A receptor**, a **ligand-gated chloride channel**, leading to increased frequency (BZDs) or duration (Barbiturates) of channel opening [1]. This enhancement of the inhibitory neurotransmitter GABA results in **CNS depression**, which is the basis for their anxiolytic, sedative, and hypnotic effects [1].***Flumazenil is the antidote for barbiturate*** **Flumazenil** is a competitive antagonist used specifically to reverse the effects of **benzodiazepines** by blocking their binding site on the GABA-A receptor [1]. There is **no specific pharmacological antidote** for barbiturate overdose; management involves supportive care like airway protection and respiratory support.***Cause additive sedation with alcohol*** Although both drugs cause severe **synergistic CNS depression** when combined with alcohol, this is a property shared by many CNS depressants, not unique to this pair, whereas their shared molecular target (D) is a fundamental defining characteristic. The combination of either BZDs or barbiturates with ethanol significantly **potentiates sedation** and increases the risk of respiratory depression and coma.***Thiopentone has short duration of action due to metabolism*** The ultra-short action of **thiopentone** following a single intravenous dose is primarily due to rapid **redistribution** out of the brain into highly perfused tissues (muscle and fat) since it is highly lipid-soluble [1, 2]. While it is eventually metabolized by the liver, **hepatic metabolism** is not the factor responsible for the swift onset and termination of its hypnotic effect [1].

Question 674: Which of the following statements is incorrect?

A. Alcohol in low dose causes brain stimulation and in higher doses, causes brain suppression
B. Cannabis use can result in self-driven, repetitive behaviours (Correct Answer)
C. Opioids are very effective analgesics
D. Volatile inhalational agents (when abused) are mostly toxic to humans

Explanation: ***Cannabis use can result in self-driven, repetitive behaviours*** - This statement is **incorrect**. Repetitive, purposeless, or **stereotyped behaviors** (**stereotypies**) are typically associated with chronic use or intoxication with **stimulants** (e.g., cocaine, amphetamines/methamphetamine), not cannabis. - Cannabis (THC) primarily acts as a depressant/hallucinogen, often causing symptoms like impaired coordination, anxiety, paranoia, altered time perception, and the **amotivational syndrome**, rather than stimulant-like stereotypies. ***Alcohol in low dose causes brain stimulation and in higher doses, causes brain suppression*** - This statement is **correct**. In low doses, alcohol causes **disinhibition** and euphoria due to selective suppression of inhibitory neurons (apparent stimulation). - In higher doses, alcohol progressively depresses the entire Central Nervous System (CNS), leading to sedation, coma, and **respiratory suppression**. ***Opioids are very effective analgesics*** - This statement is **correct**. Opioids act by agonizing **mu-opioid receptors** in the CNS, spinal cord, and peripheral nerves, causing profound **analgesia**. - They are considered the gold standard for managing severe, acute, and chronic pain, though their use is limited by addiction potential and side effects. ***Volatile inhalational agents are mostly toxic to humans*** - This statement is **correct** for recreational/abused inhalational agents (like toluene, butane, nitrites). - Abuse of these agents can cause severe immediate toxicity (e.g., **sudden sniffing death**) and long-term damage, particularly to the brain (encephalopathy), liver, and kidneys.

Question 675: A woman diagnosed with migraine has a family history of coronary artery disease. What is the drug of choice for migraine prophylaxis?

A. Topiramate
B. Ergotamine
C. Amitriptyline
D. Propranolol (Correct Answer)

Explanation: ***Propranolol*** - **Propranolol** is a first-line drug for migraine prophylaxis, particularly favored in patients with co-existing conditions like hypertension, anxiety, or, as in this case, risk factors for **Coronary Artery Disease (CAD)**. - As a non-selective beta-blocker, it provides effective migraine prevention while potentially offering **cardioprotective benefits**, making it the safest and best choice here. *Topiramate* - **Topiramate** is also a first-line prophylactic agent, but it is often preferred when patients need to avoid weight gain or have co-existing seizure disorders. - It has no particular advantage over propranolol in lowering cardiac risk and is associated with common side effects such as cognitive impairment (**'fogginess'**) and **nephrolithiasis**. *Ergotamine* - **Ergotamine preparations** are severe vasoconstrictors and are used primarily for the **acute termination** of migraine attacks, not for long-term prophylaxis. - They are absolutely **contraindicated** in patients with established or high risk of **Coronary Artery Disease** (CAD) due to the risk of **myocardial infarction** and stroke. *Amitriptyline* - **Amitriptyline** (a TCA) is used for prophylaxis, especially in patients with co-existing **insomnia** or chronic **tension headaches**. - While effective, TCAs can have potential **cardiac side effects** (e.g., QTc prolongation, orthostatic hypotension) which makes **propranolol** a medically safer choice for patients with a family history of CAD.

Question 676: Which drug acts on the receptor X shown below in skeletal muscle?

A. Dantrolene (Correct Answer)
B. Tubocurarine
C. Baclofen
D. Vecuronium

Explanation: ***Correct: Dantrolene*** - **Dantrolene** acts directly on the **ryanodine receptor (RyR1)** shown as receptor X in the sarcoplasmic reticulum of skeletal muscle - It **blocks calcium release** from the sarcoplasmic reticulum by inhibiting the ryanodine receptor - **Mechanism**: Prevents excitation-contraction coupling by reducing intracellular calcium availability - **Clinical use**: Treatment of **malignant hyperthermia** and spasticity of central origin - This is the only drug among the options that directly targets the intracellular calcium release channel *Incorrect: Tubocurarine* - Acts on **nicotinic receptors at the neuromuscular junction** (postsynaptic membrane), not on intracellular receptors - Non-depolarizing neuromuscular blocker that prevents acetylcholine binding - Does not affect calcium release from sarcoplasmic reticulum *Incorrect: Baclofen* - Acts on **GABA-B receptors** in the spinal cord (presynaptic and postsynaptic) - Reduces muscle spasticity by inhibiting excitatory neurotransmitter release at spinal level - Does not directly act on skeletal muscle calcium channels or ryanodine receptors *Incorrect: Vecuronium* - Acts on **nicotinic receptors at the neuromuscular junction**, similar to tubocurarine - Non-depolarizing neuromuscular blocker used for muscle relaxation during anesthesia - Blocks neuromuscular transmission, not intracellular calcium release mechanisms

Question 677: A patient of hemiplegic stroke is having extreme stiffness of arm and truncal musculature. Which drug marked X will act on GABA-B receptors to reduce symptoms? (Recent NEET Pattern 2016-17)

A. Dantrolene
B. Diazepam
C. Tizanidine
D. Baclofen (Correct Answer)

Explanation: ***Baclofen*** - The image shows a drug 'X' acting on **GABA-B receptors** on both pre- and post-synaptic terminals, which is the mechanism of action for **baclofen**. - Baclofen is a centrally acting **muscle relaxant** often used to treat spasticity, stiffness, and muscle spasms, consistent with the symptoms in a patient with a hemiplegic stroke. *Diazepam* - Diazepam is a **benzodiazepine** that acts as an allosteric modulator primarily on **GABA-A receptors**, increasing the frequency of chloride channel opening. - While it has muscle relaxant properties, its primary mechanism is not via GABA-B receptors, and it has more pronounced sedative effects compared to baclofen. *Tizanidine* - Tizanidine is an **alpha-2 adrenergic agonist** that reduces spasticity by increasing presynaptic inhibition of motor neurons. - It does not primarily act on GABA-B receptors, distinguishing it from the drug depicted in the image. *Dantrolene* - Dantrolene acts directly on **skeletal muscle** by inhibiting calcium release from the sarcoplasmic reticulum, thereby uncoupling excitation-contraction. - It does not work on GABA receptors in the central nervous system; its effects are peripheral.

Question 678: A 6-year-old child has poor school performance. On scolding by teacher abnormal behavior is noted. EEG is performed. Which of the following drugs will cause worsening of the patient?

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

Explanation: ***Carbamazepine*** - The EEG image shows **generalized spike-wave complexes at 3 Hz**, which are characteristic of **absence seizures** (also known as petit mal seizures). - **Carbamazepine** is known to **exacerbate absence seizures** and should be avoided in patients with this diagnosis. *Ethosuximide* - This is a **first-line drug** specifically for treating **absence seizures**. - It works by blocking **T-type calcium channels** in the thalamus, effectively reducing spike-wave discharges. *Valproate* - **Valproate** is a broad-spectrum anticonvulsant effective against various seizure types, including **absence seizures**, generalized tonic-clonic seizures, and myoclonic seizures. - It is an appropriate choice if ethosuximide is ineffective or if other seizure types coexist. *Clonazepam* - **Clonazepam** is a **benzodiazepine** that can be used as an add-on therapy for **absence seizures**, especially in refractory cases. - While it has sedative side effects, it does not typically worsen absence seizures; rather, it helps control them.

Question 679: Which anti-epileptic drug marked X will act at the site shown?

A. Tiagabine (Correct Answer)
B. Vigabatrin
C. Gabapentin
D. Rufinamide

Explanation: ***Tiagabine*** - The image shows site 'X' as a **GABA reuptake transporter** that actively removes GABA from the synaptic cleft back into the presynaptic neuron or glial cells - **Tiagabine** specifically inhibits **GABA reuptake transporters (GAT-1)**, thereby increasing GABA concentration in the synaptic cleft and enhancing its inhibitory effect - This is the mechanism directly targeting the site shown in the diagram *Vigabatrin* - Irreversible inhibitor of **GABA transaminase (GABA-T)**, the enzyme responsible for catabolizing GABA - Acts intracellularly to prevent GABA breakdown, not at the synaptic reuptake transporter shown - Different mechanism from the site depicted *Gabapentin* - GABA analog but does not bind to GABA receptors or interfere with GABA reuptake - Primary mechanism involves modulating **voltage-gated calcium channels (α2δ subunit)**, reducing excitatory neurotransmitter release - Does not act at GABA transporters *Rufinamide* - Prolongs the inactive state of **voltage-dependent sodium channels**, reducing neuronal excitability - Mechanism is distinct from GABA reuptake or metabolism - Does not act at the site shown in the diagram

Question 680: A 70-year-old man is diagnosed with Parkinsonism. Which is the drug at site marked as X?

A. Tolcapone (Correct Answer)
B. Entacapone
C. Selegiline
D. Rasagiline

Explanation: ***Tolcapone*** - The diagram shows 'X' inhibiting **COMT (catechol-O-methyltransferase)** in the periphery - **Tolcapone** is a COMT inhibitor that works both peripherally and centrally - It prevents the methylation of levodopa to 3-O-methyldopa, increasing levodopa availability to cross the blood-brain barrier - Tolcapone is more potent than entacapone but has potential hepatotoxicity risk requiring liver function monitoring *Entacapone* - Also a COMT inhibitor, but works only peripherally (not centrally) - While it does inhibit peripheral COMT like the site shown, tolcapone is the more comprehensive COMT inhibitor - Entacapone is safer with no hepatotoxicity concerns *Selegiline* - This is a selective **MAO-B (monoamine oxidase-B) inhibitor**, not a COMT inhibitor - Works by preventing dopamine breakdown in the brain - Does not act at the COMT site shown in the diagram *Rasagiline* - Also a selective **MAO-B inhibitor**, not a COMT inhibitor - Prevents dopamine degradation in the CNS - Does not inhibit COMT as shown at site X

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