Central Nervous System Pharmacology Practice Questions

Q: Tolerance is seen in all, except?

Cocaine. The correct answer is **Cocaine**. In pharmacology, tolerance refers to the requirement of higher doses of a drug to achieve the same effect previously produced by a lower dose. **Why Cocaine is the correct answer:** Cocaine is a unique CNS stimulant because it primarily exhibits **reverse tolerance (sensitization)** rather than traditional tolerance [1]. With repeated use, the user may become *more* sensitive to its effects, particularly its psychomotor and convulsant actions. While some tolerance can develop to its euphoric effects, the lack of significant physiological tolerance to its dangerous side effects (like seizures and cardiac arrhythmias) distinguishes it from the other options [3]. **Analysis of Incorrect Options:** * **Morphine (Opioids):** Exhibits profound tolerance to most effects (analgesia, euphoria, respiratory depression) except for **miosis and constipation**, which are high-yield "no-tolerance" exceptions [2]. * **Amphetamine:** Rapid tolerance (tachyphylaxis) develops to its anorectic and pressor effects due to the depletion of neuronal norepinephrine stores. * **Barbiturates:** These induce hepatic microsomal enzymes (pharmacokinetic tolerance) and cause CNS adaptation (pharmacodynamic tolerance). **High-Yield NEET-PG Pearls:** 1. **Miosis and Constipation:** Remember these two effects of Morphine never show tolerance. 2. **Tachyphylaxis:** This is a form of rapid-onset tolerance seen with Ephedrine, Tyramine, and Amphetamines. 3. **Cross-tolerance:** Often occurs between drugs of the same class (e.g., between different Benzodiazepines or between Alcohol and Barbiturates). 4. **Cocaine Mechanism:** It acts by blocking the reuptake of Dopamine, Norepinephrine, and Serotonin (NET, DAT, and SERT inhibitors) [3].

Q: Which antidepressant is relatively contraindicated in young men secondary to its side effect of priapism?

Trazodone. **Explanation:** **Trazodone** is a SARI (Serotonin Antagonist and Reuptake Inhibitor) primarily used for insomnia and depression. The correct answer is Trazodone because it is notorious for causing **priapism** (a painful, persistent erection lasting >4 hours), which is considered a urological emergency. * **Mechanism:** This side effect is attributed to its potent **$\alpha_1$-adrenergic receptor antagonism**. Blockade of these receptors in the corpora cavernosa prevents the sympathetic-mediated vasoconstriction required for detumescence, leading to "trapped" blood flow. Due to this association, it is often humorously referred to as **"Trazobone."** **Analysis of Incorrect Options:** * **Sertraline (SSRI):** The most common sexual side effect of SSRIs is **delayed ejaculation** or erectile dysfunction, not priapism. In fact, SSRIs are sometimes used therapeutically to treat premature ejaculation. * **Imipramine & Amitriptyline (TCAs):** These Tricyclic Antidepressants are more commonly associated with **anticholinergic side effects** (dry mouth, blurred vision, urinary retention) and cardiac arrhythmias. While they can cause sexual dysfunction, priapism is extremely rare compared to Trazodone. **High-Yield Clinical Pearls for NEET-PG:** * **Management of Priapism:** If Trazodone-induced priapism occurs, the immediate treatment involves intracavernosal injection of an $\alpha$-agonist like **Phenylephrine**. * **Other Side Effects:** Trazodone is highly sedating (due to $H_1$ blockade) and can cause postural hypotension (due to $\alpha_1$ blockade). * **Drug of Choice:** Trazodone is frequently used "off-label" as a hypnotic for patients with depression-associated insomnia because it lacks the addictive potential of benzodiazepines.

Q: True about phenytoin?

Causes nystagmus and ataxia. **Explanation:** Phenytoin is a high-yield hydantoin derivative used primarily for focal and generalized tonic-clonic seizures. Its mechanism involves the blockade of voltage-gated sodium channels in their inactivated state. **Why Option D is Correct:** Nystagmus and ataxia are the hallmark signs of **dose-dependent cerebellar-vestibular toxicity**. At therapeutic levels (10–20 µg/mL), mild nystagmus may appear. As levels exceed 20–30 µg/mL, ataxia, slurred speech, and incoordination develop. This occurs because phenytoin follows **zero-order kinetics** (saturation kinetics) at higher doses; a small increase in dose leads to a disproportionately large increase in plasma concentration, rapidly reaching toxic levels. **Analysis of Incorrect Options:** * **Option A:** Phenytoin has high oral bioavailability (~90%) and does **not** undergo significant first-pass metabolism. It is metabolized by hepatic CYP2C9/2C19 enzymes. * **Option B:** Phenytoin is highly **teratogenic**. It causes **Fetal Hydantoin Syndrome**, characterized by craniofacial anomalies (cleft lip/palate), microcephaly, and hypoplastic phalanges/nails. * **Option C:** Phenytoin is **ineffective** in absence seizures and may even exacerbate them. The drug of choice for absence seizures is **Ethosuximide** (or Valproate). **High-Yield Clinical Pearls for NEET-PG:** * **Non-dose dependent side effects (Mnemonic: P-H-E-N-Y-T-O-I-N):** **P**-450 induction, **H**irsutism, **E**nlarged gums (Gingival hyperplasia due to PDGF), **N**ystagmus, **Y**ellow-brown skin (pigmentation), **T**eratogenicity, **O**steomalacia (Vitamin D deficiency), **I**nterference with B12/Folate (Megaloblastic anemia), **N**europathy. * **Fosphenytoin:** A water-soluble prodrug of phenytoin used IV to avoid the local irritation and "Purple Glove Syndrome" associated with phenytoin.

Q: Which of the following antiepileptic drugs acts by affecting the levels of GABA?

Sodium valproate. **Explanation:** The correct answer is **Sodium valproate**. This drug is a broad-spectrum antiepileptic that exerts its effects through multiple mechanisms, primarily by increasing the levels of **GABA** (Gamma-Aminobutyric Acid), the brain's chief inhibitory neurotransmitter. It achieves this by: 1. Inhibiting **GABA transaminase** (the enzyme responsible for GABA degradation). 2. Stimulating **Glutamic Acid Decarboxylase (GAD)** (the enzyme responsible for GABA synthesis). 3. Blocking voltage-gated sodium channels and T-type calcium channels. **Analysis of Incorrect Options:** * **Ethosuximide:** Its primary mechanism is the inhibition of **T-type Calcium channels** in thalamic neurons. It is the drug of choice for Absence seizures but does not significantly affect GABA levels. * **Phenytoin sodium:** It acts by blocking **voltage-gated Sodium channels** in their inactive state, preventing high-frequency repetitive firing of action potentials. * **Carbamazepine:** Similar to phenytoin, it primarily works by stabilizing the inactivated state of **voltage-gated Sodium channels**. **High-Yield Clinical Pearls for NEET-PG:** * **Valproate** is the drug of choice for **Myoclonic seizures** and generalized tonic-clonic seizures (GTCS). * It is highly **teratogenic**, specifically associated with **Neural Tube Defects** (Spina Bifida); hence, folic acid supplementation is mandatory if used in pregnancy. * Other GABA-modulating drugs include **Vigabatrin** (irreversible inhibitor of GABA transaminase) and **Tiagabine** (GABA uptake inhibitor/GAT-1 blocker). * Common side effects of Valproate: Weight gain, Alopecia (curling of hair), Hepatotoxicity, and Pancreatitis.

Q: A 15-year-old boy with epilepsy, treated with a combination of valproate and phenytoin, has good seizure control. Levels of both drugs are within the therapeutic range. All of the following adverse effects can be attributed to valproate except?

Lymphadenopathy. **Explanation:** The correct answer is **Lymphadenopathy**, as it is a classic side effect associated with **Phenytoin**, not Sodium Valproate. Phenytoin can cause "Pseudolymphoma syndrome," characterized by lymph node enlargement, fever, and rash. **Analysis of Options:** * **Weight gain (Option A):** Valproate is notorious for causing significant weight gain (often >5-10 kg) due to increased appetite and metabolic changes. This is a common reason for non-compliance. * **Elevated ALT (Option B):** Hepatotoxicity is a serious adverse effect of Valproate. Asymptomatic elevation of liver enzymes (ALT/AST) occurs in up to 20% of patients. It is particularly dangerous in children under 2 years of age (Reye’s-like syndrome). * **Hyperammonemia (Option C):** Valproate interferes with the urea cycle (inhibiting carbamoyl phosphate synthetase I), leading to increased serum ammonia levels. This can occur even in the presence of normal liver function tests and may lead to encephalopathy. **High-Yield Clinical Pearls for NEET-PG:** * **Valproate Mnemonic (VALPROATE):** **V**omiting, **A**lopecia (curly hair), **L**iver toxicity, **P**ancreatitis/PCOS, **R**etention of fat (Weight gain), **O**edema, **A**norexia (rarely), **T**remors/Teratogenicity (Neural tube defects), **E**nzyme inhibitor. * **Phenytoin Specifics:** Remember **"HOT MALARIA"** for Phenytoin: **H**irsutism, **O**steomalacia, **T**eratogenicity (Fetal Hydantoin Syndrome), **M**egaloblastic anemia, **A**taxia, **L**ymphadenopathy, **I**nsulin inhibition, **R**ash, **I**nducer of CYP450, **A**lveolar hyperplasia (Gingival hyperplasia). * Valproate is a broad-spectrum anti-epileptic and is the drug of choice for Generalized Tonic-Clonic Seizures (GTCS), Myoclonic, and Absence seizures.

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

Decreases formation of 3-OMD. **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).

Q: Entacapone used in Parkinsonism is:

COMT inhibitor. **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).

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

Nephrotoxicity. **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.

Q: Dysphoria caused by opiates is mediated by which receptor?

Kappa. **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.

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

Safinamide. **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 1

Tolerance is seen in all, except?

Accepted Answer

Cocaine

Answer

Morphine

Answer

Amphetamine

Answer

Barbiturates

Question 2

Which antidepressant is relatively contraindicated in young men secondary to its side effect of priapism?

Accepted Answer

Trazodone

Answer

Sertraline

Answer

Imipramine

Answer

Amitriptyline

Question 3

True about phenytoin?

Accepted Answer

Causes nystagmus and ataxia

Answer

Undergoes first-pass metabolism

Answer

Is not teratogenic

Answer

Is the drug of choice for absence seizures

Question 4

Which of the following antiepileptic drugs acts by affecting the levels of GABA?

Accepted Answer

Sodium valproate

Answer

Ethosuximide

Answer

Phenytoin sodium

Answer

Carbamazepine

Question 5

A 15-year-old boy with epilepsy, treated with a combination of valproate and phenytoin, has good seizure control. Levels of both drugs are within the therapeutic range. All of the following adverse effects can be attributed to valproate except?

Accepted Answer

Lymphadenopathy

Answer

Weight gain of 5 kg

Answer

Serum alanine aminotransferase 150 IU/L

Answer

Rise in serum ammonia level by 20 µmol/L

Question 6

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

Accepted Answer

Decreases formation of 3-OMD

Answer

Activates COMT

Answer

Inhibits monoamine oxidase type B

Answer

Inhibits dopamine uptake

Question 7

Entacapone used in Parkinsonism is:

Accepted Answer

COMT inhibitor

Answer

Dopamine agonist

Answer

MAO inhibitor

Answer

Dopa decarboxylase inhibitor

Question 8

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

Accepted Answer

Nephrotoxicity

Answer

Alopecia

Answer

Hepatitis

Answer

Skin rashes

Question 9

Dysphoria caused by opiates is mediated by which receptor?

Accepted Answer

Kappa

Answer

mu

Answer

Delta

Answer

Sigma

Question 10

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

Accepted Answer

Safinamide

Answer

Selegiline

Answer

Rasagiline

Answer

All of the above

Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology — MCQs

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