Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 391: Entacapone may be useful in patients being treated with levodopa-carbidopa combination because it:

A. Activates COMT
B. Decreases formation of 3-OMD (Correct Answer)
C. Inhibits monoamine oxidase type B
D. Inhibits dopamine uptake

Explanation: **Explanation:** **1. Why Option B is Correct:** Entacapone is a selective and reversible **COMT (Catechol-O-Methyltransferase) inhibitor**. When Levodopa is administered with Carbidopa (a peripheral DOPA decarboxylase inhibitor), the decarboxylation pathway is blocked. This shifts the metabolism of Levodopa toward the COMT pathway, leading to the formation of **3-O-methyldopa (3-OMD)**. High levels of 3-OMD are problematic because it competes with Levodopa for the same large neutral amino acid (LNAA) carrier system to cross the blood-brain barrier. By inhibiting peripheral COMT, Entacapone **decreases the formation of 3-OMD**, thereby increasing the bioavailability of Levodopa and its transport into the CNS. **2. Why Other Options are Incorrect:** * **Option A:** Entacapone **inhibits** COMT; it does not activate it. * **Option C:** Inhibition of MAO-B is the mechanism of drugs like **Selegiline and Rasagiline**, not Entacapone. * **Option D:** Inhibition of dopamine uptake (reuptake inhibition) is a property of drugs like **Amantadine** or certain antidepressants, but not COMT inhibitors. **3. NEET-PG High-Yield Pearls:** * **Entacapone vs. Tolcapone:** Entacapone acts only **peripherally**, whereas Tolcapone acts both **peripherally and centrally**. * **Hepatotoxicity:** Tolcapone is associated with severe hepatotoxicity (requires LFT monitoring); Entacapone is generally safer and preferred. * **Clinical Use:** These drugs are used to manage **"wearing-off" phenomena** (end-of-dose akinesia) in Parkinson’s patients. * **Side Effect:** A characteristic side effect of COMT inhibitors is **orange-colored urine** (harmless).

Question 392: Entacapone used in Parkinsonism is:

A. Dopamine agonist
B. MAO inhibitor
C. COMT inhibitor (Correct Answer)
D. Dopa decarboxylase inhibitor

Explanation: **Entacapone** is a selective and reversible **COMT (Catechol-O-methyltransferase) inhibitor** [1, 3]. In Parkinson’s disease, when Levodopa is administered, it is rapidly metabolized by COMT in the periphery into 3-O-methyldopa (3-OMD) [1, 3]. Entacapone inhibits this peripheral metabolism, thereby increasing the bioavailability of Levodopa and prolonging its half-life [1, 3]. This helps in managing the "wearing-off" phenomenon and motor fluctuations [2]. **Analysis of Options:** * **Option A (Dopamine agonist):** These drugs (e.g., Bromocriptine, Pramipexole, Ropinirole) directly stimulate dopamine receptors. Entacapone does not act on receptors; it modifies metabolism. * **Option B (MAO inhibitor):** MAO-B inhibitors (e.g., Selegiline, Rasagiline) prevent the breakdown of dopamine within the CNS. While they also prolong dopamine action, Entacapone specifically targets the COMT enzyme. * **Option D (Dopa decarboxylase inhibitor):** Drugs like Carbidopa and Benserazide inhibit the conversion of Levodopa to Dopamine in the periphery. Entacapone is often used as an adjunct to the Levodopa+Carbidopa combination. **High-Yield Clinical Pearls for NEET-PG:** * **Entacapone vs. Tolcapone:** Entacapone acts only **peripherally**, whereas Tolcapone acts both **centrally and peripherally** [2]. * **Toxicity:** Tolcapone is associated with severe **hepatotoxicity** (requires LFT monitoring), while Entacapone is not. * **Side Effect:** A characteristic side effect of Entacapone is the **orange-discoloration of urine**. * **Triple Therapy:** The combination of Levodopa + Carbidopa + Entacapone is available as a single formulation (Stalevo).

Question 393: All of the following are side effects of valproic acid, except?

A. Alopecia
B. Hepatitis
C. Nephrotoxicity (Correct Answer)
D. Skin rashes

Explanation: **Explanation:** Valproic acid is a broad-spectrum antiepileptic drug that acts by multiple mechanisms, including sodium channel blockade, potentiation of GABAergic activity, and inhibition of T-type calcium channels. **Why Nephrotoxicity is the Correct Answer:** Valproic acid is primarily metabolized by the liver through glucuronidation and mitochondrial beta-oxidation. It is **not** associated with renal damage or nephrotoxicity. In contrast, drugs like Lithium or Aminoglycosides are classic examples of nephrotoxic agents in pharmacology. **Analysis of Incorrect Options:** * **Alopecia:** This is a common, reversible side effect of valproate. It may also cause thinning of hair or the development of "curly hair" upon regrowth. * **Hepatitis:** Hepatotoxicity is a rare but serious idiosyncratic side effect, especially in children under two years of age taking multiple anticonvulsants. Monitoring Liver Function Tests (LFTs) is essential. * **Skin Rashes:** While less common than with Phenytoin or Lamotrigine, valproic acid can cause hypersensitivity reactions, including skin rashes and, rarely, Stevens-Johnson Syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonics for Valproate Side Effects (VALPROATE):** **V**omit, **A**lopecia, **L**iver toxicity, **P**ancreatitis/PCOS, **R**etention of weight (Weight gain), **O**edema, **A**taxia, **T**eratogenicity (Neural Tube Defects), **E**nzyme inhibitor. * **Teratogenicity:** It is the most teratogenic antiepileptic, specifically causing **Spina Bifida**. * **Metabolism:** It is a potent **Enzyme Inhibitor**, which increases the plasma concentration of other drugs like Phenobarbital and Lamotrigine. * **Drug of Choice:** It remains the drug of choice for Generalized Tonic-Clonic Seizures (GTCS), Myoclonic seizures, and Absence seizures.

Question 394: Dysphoria caused by opiates is mediated by which receptor?

A. mu
B. Kappa (Correct Answer)
C. Delta
D. Sigma

Explanation: **Explanation:** The correct answer is **Kappa (κ) receptors**. Opioid receptors are G-protein coupled receptors (GPCRs) located throughout the central nervous system, and their activation leads to distinct pharmacological profiles. 1. **Why Kappa is correct:** Activation of Kappa receptors (specifically by agonists like pentazocine or endogenous dynorphins) is uniquely associated with **dysphoria**, hallucinations, and psychotomimetic effects. This occurs because Kappa activation inhibits dopamine release in the mesolimbic pathway, contrasting with the "reward" sensation of other opioids. 2. **Why other options are incorrect:** * **Mu (μ):** These are the primary receptors for most clinical opioids (e.g., Morphine). Their activation causes **euphoria**, supraspinal analgesia, respiratory depression, and physical dependence. * **Delta (δ):** These receptors primarily mediate spinal/supraspinal analgesia and may have antidepressant-like effects; they are not associated with dysphoria. * **Sigma (σ):** Formerly classified as an opioid receptor, it is now considered a non-opioid site. While its activation can cause hallucinations and mydriasis, it is not the mediator of classic opioid-induced dysphoria. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **M**u = **M**ore Euphoria; **K**appa = **K**atastrophic (Dysphoria). * **Dynorphins** are the endogenous ligands with the highest affinity for Kappa receptors. * **Mixed Agonist-Antagonists** (like Pentazocine and Butorphanol) act as Kappa agonists and Mu antagonists/partial agonists; they are notorious for causing dysphoria as a side effect. * **Miosis** (pinpoint pupils) is mediated by both Mu and Kappa receptors via the Edinger-Westphal nucleus.

Question 395: Which of the following is a reversible MAO-B inhibitor used in the treatment of Parkinsonism?

A. Selegiline
B. Safinamide (Correct Answer)
C. Rasagiline
D. All of the above

Explanation: **Explanation:** The correct answer is **Safinamide**. Monoamine oxidase type B (MAO-B) inhibitors are used in Parkinson’s disease to prevent the breakdown of dopamine in the striatum. The key distinction in this question lies in the **reversibility** of the inhibition. 1. **Safinamide (Correct):** It is a unique, highly selective, and **reversible** MAO-B inhibitor. In addition to its MAO-B effects, it also inhibits glutamate release and blocks voltage-gated sodium and calcium channels, providing both dopaminergic and non-dopaminergic benefits. 2. **Selegiline (Incorrect):** This is an **irreversible** MAO-B inhibitor. It is metabolized into amphetamine and methamphetamine, which can cause side effects like insomnia and jitteriness. 3. **Rasagiline (Incorrect):** This is also an **irreversible** MAO-B inhibitor. It is more potent than selegiline and is not metabolized into amphetamine derivatives, making it better tolerated. **High-Yield Clinical Pearls for NEET-PG:** * **The "Cheese Reaction":** While selective MAO-B inhibitors (at standard doses) usually do not cause hypertensive crises with tyramine-rich foods, the risk increases if selectivity is lost at high doses. * **Drug Interactions:** Avoid combining MAO-B inhibitors with SSRIs, SNRIs, or Meperidine due to the risk of **Serotonin Syndrome**. * **Clinical Use:** These drugs are used as monotherapy in early Parkinson’s or as adjuncts to Levodopa to reduce "off" time in advanced stages. * **Mnemonic:** Remember **S**afinamide is **S**elective and **S**hort-acting (Reversible), whereas Selegiline and Rasagiline are "Permanent" (Irreversible) binders.

Question 396: Which of the following is NOT a pharmacological effect of benzodiazepines?

A. Anxiolysis
B. Sedation
C. Retrograde amnesia (Correct Answer)
D. Anticonvulsant action

Explanation: **Explanation:** Benzodiazepines (BZDs) act by enhancing the effect of the inhibitory neurotransmitter GABA at the **GABA-A receptor**. By increasing the frequency of chloride channel opening, they cause hyperpolarization of neurons, leading to various CNS effects. **Why "Retrograde Amnesia" is the correct answer:** Benzodiazepines cause **Anterograde amnesia** (inability to form new memories after taking the drug), which is clinically useful for unpleasant medical procedures like endoscopies. They **do not** cause **Retrograde amnesia** (loss of memories formed before the drug administration). This is a common "trap" question in competitive exams. **Analysis of Incorrect Options:** * **A. Anxiolysis:** At low doses, BZDs selectively inhibit circuits in the limbic system, making them effective for treating generalized anxiety and panic disorders. * **B. Sedation:** BZDs exert a calming effect and induce sleep (hypnosis) by depressing the CNS, though they reduce REM sleep duration. * **D. Anticonvulsant action:** By enhancing GABAergic inhibition, BZDs suppress the spread of seizure activity. Drugs like Diazepam and Lorazepam are first-line for Status Epilepticus. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** BZDs increase the **frequency** of chloride channel opening (Barbiturates increase the **duration**). * **Antidote:** **Flumazenil** is a competitive BZD antagonist used for overdose. * **Metabolism:** Most BZDs are metabolized by the liver. **LOT** (Lorazepam, Oxazepam, Temazepam) are preferred in elderly patients or those with liver failure as they undergo direct conjugation. * **Muscle Relaxation:** BZDs also provide central muscle relaxation via action on the spinal cord.

Question 397: All of the following drugs can be used for the prophylaxis of migraine except?

A. Propranolol
B. Sumatriptan (Correct Answer)
C. Amitriptyline
D. Flunarizine

Explanation: **Explanation:** The management of migraine is divided into two categories: **Abortive (Acute) treatment** and **Prophylactic (Preventive) treatment**. **Why Sumatriptan is the correct answer:** Sumatriptan is a selective **5-HT$_{1B/1D}$ receptor agonist**. Its primary mechanism involves vasoconstriction of dilated cranial blood vessels and inhibition of neuropeptide release from trigeminal nerve endings. Because of its rapid onset and short duration of action, it is used exclusively for the **acute abortive treatment** of migraine attacks. It has no role in prophylaxis and, if overused, can actually lead to "Medication Overuse Headache." **Analysis of Incorrect Options (Prophylactic Agents):** * **Propranolol (Option A):** A non-selective beta-blocker and the **first-line drug** for migraine prophylaxis. It works by stabilizing vascular tone and increasing the threshold for migraine triggers. * **Amitriptyline (Option C):** A Tricyclic Antidepressant (TCA) that is highly effective for prophylaxis, especially in patients with co-existing tension-type headaches, depression, or insomnia. * **Flunarizine (Option D):** A non-selective **Calcium Channel Blocker** (CCB) with H1-blocking activity. It is specifically used for prophylaxis due to its long half-life, though it may cause weight gain and sedation. **High-Yield NEET-PG Pearls:** 1. **DOC for Acute Attack:** Mild (NSAIDs/Paracetamol); Moderate-Severe (Triptans). 2. **DOC for Prophylaxis:** Propranolol (most common) or Amitriptyline. 3. **Valproate and Topiramate** are the preferred anti-epileptics for migraine prevention. 4. **CGRP Antagonists (Erenumab):** Newer monoclonal antibodies used for refractory prophylaxis. 5. **Contraindication for Triptans:** Ischemic heart disease or Prinzmetal angina (due to coronary vasospasm).

Question 398: Which of the following is a gender-specific adverse effect of valproate?

A. Weight gain
B. Tremors
C. Alopecia
D. Polycystic ovarian disease (Correct Answer)

Explanation: **Explanation:** **Sodium Valproate** is a broad-spectrum antiepileptic drug that acts by increasing GABA levels, inhibiting sodium channels, and blocking T-type calcium channels. **Why Polycystic Ovarian Disease (PCOD) is the correct answer:** Valproate is strongly associated with endocrine disruptions in females. It can cause **hyperandrogenism** and interfere with the hypothalamic-pituitary-ovarian axis, leading to menstrual irregularities, hirsutism, and the development of **Polycystic Ovarian Disease (PCOD)**. This is a gender-specific adverse effect as it specifically impacts the female reproductive system. Due to this risk, along with its high teratogenic potential (neural tube defects), valproate is generally avoided in women of childbearing age unless no other alternative exists. **Analysis of Incorrect Options:** * **A. Weight gain:** This is a very common side effect of valproate, but it occurs in both males and females (not gender-specific). * **B. Tremors:** Fine tremors are a dose-related side effect seen in a significant percentage of patients regardless of gender. * **C. Alopecia:** Transient hair loss or thinning (often resulting in "curly hair" regrowth) is a known side effect but affects both sexes. **High-Yield NEET-PG Pearls:** * **Teratogenicity:** Valproate is the most teratogenic antiepileptic, specifically causing **Spina Bifida** (Neural tube defects). * **Mnemonic for Valproate Side Effects (VALPROATE):** **V**omiting, **A**lopecia, **L**iver toxicity (Hepatotoxicity - most serious), **P**ancreatitis/PCOD, **R**etention of weight (Weight gain), **O**edema, **A**norexia, **T**remors/Teratogenicity, **E**nzyme inhibitor (Note: It is a potent microsomal enzyme inhibitor, unlike most other AEDs which are inducers).

Question 399: All of the following are true about cocaine except one?

A. Is an ester of benzoic acid
B. Produces para-aminobenzoic acid as a metabolite (Correct Answer)
C. Is metabolized by the liver
D. Is a vasoconstrictor

Explanation: ### Explanation **Correct Option: B. Produces para-aminobenzoic acid as a metabolite** Cocaine is unique among local anesthetics. While it is chemically an **ester**, it is an ester of **benzoic acid**, not para-aminobenzoic acid (PABA). Most other ester-linked local anesthetics (like procaine or benzocaine) are derivatives of PABA and release it upon hydrolysis. PABA is clinically significant because it can cause allergic reactions and antagonize the effects of sulfonamides. Since cocaine does not produce PABA, it does not share this specific allergenic profile. **Analysis of Other Options:** * **A. Is an ester of benzoic acid:** This is true. Cocaine is a naturally occurring alkaloid derived from the coca plant and is chemically classified as an ester of benzoic acid and ecgonine. * **C. Is metabolized by the liver:** This is true. Although cocaine is primarily hydrolyzed by plasma pseudocholinesterase, it also undergoes significant metabolism by **liver esterases** to form metabolites like benzoylecgonine (the substance usually detected in urine drug screens). * **D. Is a vasoconstrictor:** This is true. Cocaine is the **only local anesthetic** that inherently causes vasoconstriction. It achieves this by blocking the reuptake of norepinephrine (NET inhibition) at sympathetic nerve endings, leading to localized ischemia and shrinkage of mucous membranes. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Blocks voltage-gated $Na^+$ channels (local anesthetic effect) and inhibits reuptake of Catecholamines (sympathomimetic effect). * **Clinical Use:** Primarily used topically in ENT surgeries (e.g., dacryocystorhinostomy) for its combined anesthetic and hemostatic properties. * **Toxicity:** Can cause hypertension, tremors, seizures, and myocardial infarction (due to coronary vasospasm). * **Drug of Abuse:** It produces intense euphoria by increasing dopamine levels in the nucleus accumbens (reward pathway).

Question 400: Ethosuxamide is the drug of choice for which type of seizure disorder?

A. Generalized tonic-clonic seizures
B. Complex partial seizures
C. Absence seizures (Correct Answer)
D. Myoclonic seizures

Explanation: ### Explanation **Correct Answer: C. Absence seizures** **Mechanism and Rationale:** Ethosuximide is the first line drug of choice for **Absence seizures (Petit mal)**. The underlying pathophysiology of absence seizures involves abnormal T-type calcium channel activity in thalamic neurons, which leads to the characteristic 3 Hz spikend-wave discharges on an EEG. Ethosuximide works specifically by **inhibiting these T-type calcium channels**, thereby stabilizing the thalamocortical excitability. **Analysis of Incorrect Options:** * **A. Generalized tonic-clonic seizures (GTCS):** Ethosuximide is ineffective against GTCS. The drugs of choice for GTCS are Valproate, Levetiracetam, or Phenytoin [1]. * **B. Complex partial seizures:** These are now classified as Focal Impaired Awareness seizures. Carbamazepine or Levetiracetam are typically preferred. Ethosuximide has a very narrow spectrum and does not work for focal seizures [1], [2]. * **D. Myoclonic seizures:** Sodium Valproate is the drug of choice for myoclonic seizures. Ethosuximide is strictly limited to absence seizures. **NEET-PG High-Yield Pearls:** * **The "Valproate vs. Ethosuximide" Rule:** While Ethosuximide is the drug of choice for *pure* absence seizures, **Sodium Valproate** is preferred if the patient has *mixed* seizure types (e.g., Absence + GTCS) [1]. * **Side Effects:** Remember the mnemonic **EFGHIJ**: **E**thosuximide causes **F**atigue, **G**I distress, **H**eadache, **I**tching (Urticaria), and **J**-Steven **J**ohnson Syndrome. * **EEG Finding:** Absence seizures are classically associated with a **3 Hz spike-and-wave pattern**, which is a frequent "image-based" or clinical vignette trigger in exams.

Practice by Chapter

General Anesthetics

Practice Questions

Local Anesthetics

Practice Questions

Sedative-Hypnotics

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Antiepileptic Drugs

Practice Questions

Antiparkinsonian Drugs

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Opioid Analgesics

Practice Questions

Drugs of Abuse and Addiction

Practice Questions

Psychostimulants

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Hallucinogens

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CNS Stimulants and Cognitive Enhancers

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