Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 441: Which of the following can be used in the treatment of myoclonic seizures, except?

A. Valproate
B. Carbamazepine (Correct Answer)
C. Topiramate
D. Zonisamide

Explanation: **Explanation:** The correct answer is **Carbamazepine**. **1. Why Carbamazepine is the correct answer:** In the treatment of **Myoclonic seizures**, certain sodium channel blockers like **Carbamazepine**, **Phenytoin**, and **Oxcarbazepine** are strictly contraindicated. These drugs can paradoxically **exacerbate** or worsen myoclonic and absence seizures. The underlying mechanism involves the enhancement of GABA-mediated inhibitory currents in the thalamocortical circuit or the specific blockade of sodium channels that may inadvertently increase the synchrony of neuronal firing in these specific seizure types. **2. Why the other options are incorrect:** * **Valproate (Option A):** This is the **drug of choice** for myoclonic seizures. It has a broad spectrum of activity, affecting sodium channels, T-type calcium channels, and increasing GABA levels. * **Topiramate (Option C):** A broad-spectrum anti-epileptic drug (AED) that is effective against multiple seizure types, including myoclonic seizures, due to its multiple mechanisms (Na+ channel blockade, AMPA/Kainate antagonism, and GABA enhancement). * **Zonisamide (Option D):** Another broad-spectrum agent that is effective in treating myoclonic jerks, particularly in progressive myoclonic epilepsies. **3. High-Yield Clinical Pearls for NEET-PG:** * **Broad-spectrum AEDs** (Valproate, Levetiracetam, Topiramate, Zonisamide, Lamotrigine) are generally safe for myoclonic seizures. * **Narrow-spectrum AEDs** (Carbamazepine, Phenytoin, Gabapentin, Pregabalin, Vigabatrin) should be **avoided** in myoclonic and absence seizures as they can worsen the condition. * **Drug of Choice (DOC) Summary:** * Myoclonic Seizures: **Valproate** * Absence Seizures: **Ethosuximide** (Valproate if generalized tonic-clonic seizures are also present). * Trigeminal Neuralgia: **Carbamazepine**.

Question 442: Galantamine is used in which condition?

A. Alzheimer's disease (Correct Answer)
B. Parkinson's disease
C. Emesis
D. Chorea

Explanation: **Explanation:** **Correct Answer: A. Alzheimer's disease** **Mechanism of Action:** Galantamine is a **reversible, competitive acetylcholinesterase (AChE) inhibitor**. In Alzheimer’s disease, there is a progressive loss of cholinergic neurons in the nucleus basalis of Meynert, leading to a deficiency of acetylcholine (ACh). By inhibiting the enzyme that breaks down ACh, Galantamine increases the concentration of the neurotransmitter at the synaptic cleft, thereby improving cognitive function. Additionally, Galantamine acts as an **allosteric modulator of nicotinic receptors**, further enhancing cholinergic transmission. **Why other options are incorrect:** * **B. Parkinson’s disease:** This condition is characterized by dopamine deficiency. Treatment involves dopamine agonists (Levodopa) or anticholinergics (Benztropine) to balance the relative cholinergic overactivity. AChE inhibitors like Galantamine would worsen Parkinsonian symptoms. * **C. Emesis:** AChE inhibitors often *cause* nausea and vomiting as side effects due to increased peripheral cholinergic activity. Antiemetics typically target D2, 5-HT3, or H1 receptors. * **D. Chorea:** Huntington’s chorea is managed by depleting dopamine (e.g., Tetrabenazine) or using antipsychotics. Galantamine has no therapeutic role in hyperkinetic movement disorders. **High-Yield Clinical Pearls for NEET-PG:** * **FDA-approved drugs for Alzheimer’s:** Donepezil, Rivastigmine, and Galantamine (Cholinesterase inhibitors) are used for mild-to-moderate cases. **Memantine** (NMDA antagonist) is used for moderate-to-severe cases. * **Rivastigmine** is unique because it is available as a **transdermal patch** and is also approved for Parkinson’s Disease Dementia. * **Side Effects:** Common side effects of Galantamine include GI distress (nausea, diarrhea), bradycardia, and syncope.

Question 443: On which subunit of the GABA receptor does benzodiazepine bind?

A. gamma-subunit
B. alpha-subunit (Correct Answer)
C. beta-subunit
D. delta-subunit

Explanation: **Explanation:** The **GABA-A receptor** is a pentameric ligand-gated chloride channel, typically composed of two $\alpha$, two $\beta$, and one $\gamma$ subunit. **Why Alpha-subunit is correct:** Benzodiazepines (BZDs) bind to a specific pocket located at the **interface of the $\alpha$ and $\gamma$ subunits**. However, for NEET-PG purposes, the binding site is functionally defined by the **$\alpha$-subunit**. Specifically, BZDs bind to $\alpha_1, \alpha_2, \alpha_3,$ and $\alpha_5$ subunits. * **$\alpha_1$ subunit:** Mediates sedation, hypnosis, and amnesia. * **$\alpha_2$ and $\alpha_3$ subunits:** Mediate anxiolytic and muscle relaxant effects. **Analysis of Incorrect Options:** * **Beta ($\beta$) subunit:** This is the primary binding site for **GABA** (the endogenous neurotransmitter) and **Barbiturates**. Barbiturates increase the *duration* of chloride channel opening, whereas BZDs increase the *frequency*. * **Gamma ($\gamma$) subunit:** While the $\gamma_2$ subunit is essential for creating the BZD binding pocket, the pharmacological specificity and action are attributed to the $\alpha$ subunit. * **Delta ($\delta$) subunit:** These are usually found in extrasynaptic GABA receptors and are sensitive to neurosteroids and alcohol, rather than BZDs. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism:** BZDs are **positive allosteric modulators**; they require GABA to be present to function (unlike barbiturates, which at high doses can act as GABA-mimetics). 2. **Antagonist:** **Flumazenil** is a competitive antagonist that binds to the same $\alpha$-interface to reverse BZD overdose. 3. **Z-drugs (Zolpidem, Zaleplon):** These are selective for the **$\alpha_1$ subunit** only, explaining why they are used for sleep without significant anxiolytic effects.

Question 444: Pseudolymphoma can result from long-term use of which medication?

A. Phenytoin (Correct Answer)
B. Carbamazepine
C. Sodium valproate
D. Phenobarbital

Explanation: **Explanation:** **Phenytoin** is a well-known cause of **Pseudolymphoma** (also known as Phenytoin-induced lymphadenopathy). This condition is a hypersensitivity reaction characterized by fever, skin rash, and generalized lymphadenopathy that clinically and histologically mimics Hodgkin’s or non-Hodgkin’s lymphoma. The mechanism involves a drug-induced immune dysregulation leading to benign lymphoid hyperplasia. Crucially, this condition is reversible and typically resolves within weeks of discontinuing the drug. **Analysis of Options:** * **Phenytoin (Correct):** It is the classic antiepileptic associated with pseudolymphoma. It is also linked to a more severe (though rare) progression to true malignant lymphoma. * **Carbamazepine:** While it can cause hypersensitivity syndromes (like DRESS), it is not the primary or classic association for pseudolymphoma in medical literature compared to Phenytoin. * **Sodium Valproate:** Common side effects include weight gain, hepatotoxicity, hair loss (alopecia), and pancreatitis, but not pseudolymphoma. * **Phenobarbital:** Primarily causes sedation, cognitive impairment, and skin rashes (Stevens-Johnson Syndrome), but is not associated with lymphadenopathy syndromes. **High-Yield Clinical Pearls for NEET-PG:** * **Phenytoin Side Effects (Mnemonic: HOT MALAI):** **H**irsutism, **O**steomalacia, **T**eratogenicity (Fetal Hydantoin Syndrome), **M**egaloblastic anemia (due to folate deficiency), **A**taxia/Nystagmus, **L**ymphadenopathy (Pseudolymphoma), **A**rrhythmias (on rapid IV), **I**nsulin inhibition (Hyperglycemia), and **Gum Hyperplasia**. * **Zero-order kinetics:** Phenytoin follows saturation kinetics at therapeutic doses, making its plasma levels unpredictable. * **Drug of Choice:** Phenytoin remains a first-line agent for Status Epilepticus (after Benzodiazepines) and Generalized Tonic-Clonic Seizures (GTCS).

Question 445: Valproic acid causes all EXCEPT-

A. It is an enzyme inducer (Correct Answer)
B. It causes obesity
C. It causes hirsutism
D. It causes neural tube defects

Explanation: **Explanation:** The correct answer is **A (It is an enzyme inducer)** because Valproic acid is actually a potent **microsomal enzyme inhibitor**. Unlike most older antiepileptics (like Phenytoin, Carbamazepine, and Phenobarbitone) which induce CYP450 enzymes, Valproic acid inhibits them. This leads to significant drug interactions, such as increasing the plasma levels of Phenobarbital and Lamotrigine when co-administered. **Analysis of other options:** * **B (Obesity):** Weight gain is a very common side effect of Valproate therapy, often leading to metabolic issues and decreased compliance. * **C (Hirsutism):** This is a tricky point in pharmacology. While Valproate is classically associated with **alopecia** (hair loss/thinning), it is also a key cause of **Polycystic Ovarian Syndrome (PCOS)** in female patients. PCOS presents with hormonal imbalances that lead to **hirsutism** and acne. * **D (Neural tube defects):** Valproate is highly teratogenic. It interferes with folate metabolism, specifically increasing the risk of **Spina Bifida** (neural tube defects) if taken during pregnancy. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Valproate Side Effects (VALPROATE):** **V**omiting, **A**lopecia, **L**iver toxicity (Hepatotoxicity), **P**ancreatitis/PCOS, **R**etention of fat (Obesity), **O**edema, **A**norexia, **T**eratogenicity, **E**nzyme inhibitor. * **Drug of Choice:** Valproate is the drug of choice for **Myoclonic seizures** and is a broad-spectrum agent used for Generalized Tonic-Clonic Seizures (GTCS) and Absence seizures. * **Hepatotoxicity:** It is contraindicated in children under 2 years due to the risk of fatal fulminant hepatitis.

Question 446: What is the primary classification of Baclofen?

A. Centrally acting muscle relaxant (Correct Answer)
B. Peripherally acting muscle relaxant
C. Both centrally and peripherally acting muscle relaxant
D. Direct acting muscle relaxant

Explanation: **Explanation:** **Baclofen** is a structural analog of the inhibitory neurotransmitter Gamma-Aminobutyric Acid (GABA). It is classified as a **centrally acting muscle relaxant** because its primary site of action is within the spinal cord. **Mechanism of Action:** Baclofen acts as a selective **GABA-B receptor agonist**. By stimulating these receptors in the dorsal horn of the spinal cord, it induces hyperpolarization. This leads to: 1. **Presynaptic inhibition:** Reducing the release of excitatory neurotransmitters (like glutamate and aspartate). 2. **Postsynaptic inhibition:** Reducing the excitability of alpha motor neurons. The result is a reduction in the frequency and severity of muscle spasms. **Analysis of Options:** * **Option A (Correct):** It acts on the CNS (spinal cord) to reduce spasticity. * **Option B (Incorrect):** Peripherally acting relaxants (e.g., Succinylcholine, Vecuronium) act at the Neuromuscular Junction (NMJ), not the spinal cord. * **Option C (Incorrect):** Baclofen does not have a direct clinical effect on the motor endplate or muscle fibers. * **Option D (Incorrect):** **Dantrolene** is the classic "direct-acting" muscle relaxant, as it acts on the Ryanodine receptors (RyR1) of the sarcoplasmic reticulum within the muscle cell. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Baclofen is the preferred agent for treating spasticity associated with **Multiple Sclerosis** and spinal cord injuries. * **Route:** Can be administered orally or via an **intrathecal pump** for severe cases. * **Side Effects:** Drowsiness, fatigue, and muscle weakness. * **Withdrawal Warning:** Abrupt discontinuation can lead to visual hallucinations, seizures, and rebound spasticity; it must be tapered gradually.

Question 447: Which of the following is not a side effect of Quetiapine?

A. Diarrhoea (Correct Answer)
B. Weight gain
C. Somnolence
D. Blurring of vision

Explanation: **Explanation:** Quetiapine is a second-generation (atypical) antipsychotic that acts as an antagonist at multiple receptors, including Dopamine (D2), Serotonin (5-HT2A), Histamine (H1), and Alpha-1 adrenergic receptors. It also possesses significant **anticholinergic** properties. **Why Diarrhoea is the correct answer:** Quetiapine, like many antipsychotics, has potent anticholinergic effects. Anticholinergic activity leads to decreased gastrointestinal motility, which typically causes **constipation**, not diarrhoea. Therefore, diarrhoea is not a characteristic side effect of this drug. **Analysis of Incorrect Options:** * **Weight Gain:** Atypical antipsychotics are notorious for metabolic side effects. Quetiapine causes significant weight gain and dyslipidemia, primarily due to its H1 and 5-HT2C receptor antagonism. * **Somnolence:** This is the most common side effect of Quetiapine. It is caused by its potent **H1-receptor blockade** (antihistaminic effect). Because of this, it is often used off-label as a sedative. * **Blurring of Vision:** This is a classic **anticholinergic** side effect resulting from the blockade of muscarinic receptors, leading to cycloplegia (paralysis of accommodation). **High-Yield Clinical Pearls for NEET-PG:** * **"Cataract Risk":** Historically, Quetiapine was associated with lens changes; though less common now, periodic eye exams are often mentioned in boards. * **Lowest EPS:** Quetiapine has one of the lowest risks of Extrapyramidal Side Effects (EPS) and hyperprolactinemia among antipsychotics, making it a preferred choice in **Parkinson’s Disease** patients with psychosis. * **Metabolic Syndrome:** Always monitor blood glucose and lipid profiles when a patient is on Quetiapine.

Question 448: Which drug is a specific MAO-B inhibitor?

A. Selegiline (Correct Answer)
B. Phenelzine
C. Tranylcypromine
D. Haloperidol

Explanation: **Explanation:** **Selegiline** is a selective and irreversible inhibitor of **Monoamine Oxidase-B (MAO-B)**. In the brain, MAO-B is the primary enzyme responsible for the degradation of dopamine. By inhibiting this enzyme, Selegiline increases dopamine levels in the nigrostriatal pathway, making it a valuable adjunct in the management of **Parkinson’s Disease**. At conventional doses, it spares MAO-A, thereby avoiding the "cheese reaction" (hypertensive crisis) associated with non-selective MAO inhibitors. **Analysis of Incorrect Options:** * **Phenelzine & Tranylcypromine:** These are **non-selective, irreversible MAO inhibitors** that inhibit both MAO-A and MAO-B. They are primarily used as antidepressants but require strict dietary restrictions (avoidance of tyramine-rich foods) to prevent hypertensive crises. * **Haloperidol:** This is a typical (first-generation) **antipsychotic** that acts as a potent **D2 receptor antagonist**. It has no inhibitory effect on the MAO enzyme; in fact, its mechanism is functionally opposite to drugs used in Parkinsonism. **High-Yield Clinical Pearls for NEET-PG:** * **Rasagiline** is another potent, irreversible, selective MAO-B inhibitor often considered more potent than Selegiline. * **Metabolism:** Selegiline is metabolized into **amphetamine and methamphetamine**, which may cause insomnia if taken late in the day. * **Cheese Reaction:** This occurs when non-selective MAOIs prevent the breakdown of dietary tyramine, leading to massive norepinephrine release. Selective MAO-B inhibitors (at low doses) do not typically cause this. * **Serotonin Syndrome:** Caution must be exercised when combining MAO inhibitors with SSRIs or TCAs.

Question 449: Which centrally acting muscle relaxant also possesses alpha-2 adrenergic agonist activity?

A. Chlorzoxazone
B. Baclofen
C. Tizanidine (Correct Answer)
D. Pirenzepine

Explanation: **Explanation:** **Tizanidine** is the correct answer because it is a centrally acting muscle relaxant that specifically acts as a **selective alpha-2 ($\alpha_2$) adrenergic agonist**. It is structurally related to clonidine but possesses significantly less (about 1/10th to 1/50th) antihypertensive potency. Its primary mechanism involves stimulating presynaptic $\alpha_2$ receptors in the spinal cord, which inhibits the release of excitatory amino acids (like glutamate and aspartate) from spinal interneurons. This action reduces polysynaptic reflex activity, thereby decreasing muscle spasticity. **Analysis of Incorrect Options:** * **Chlorzoxazone (A):** A centrally acting muscle relaxant that works primarily at the level of the spinal cord and subcortical areas of the brain to inhibit multisynaptic reflex arcs. It does not have $\alpha_2$ agonist activity. * **Baclofen (B):** A structural analog of GABA that acts as a selective **GABA-B receptor agonist**. It induces muscle relaxation by increasing potassium conductance, leading to hyperpolarization of motor neurons. * **Pirenzepine (D):** An **M1-selective anticholinergic** drug formerly used to reduce gastric acid secretion in peptic ulcer disease. It is not a muscle relaxant. **NEET-PG High-Yield Pearls:** * **Tizanidine Side Effects:** Drowsiness, xerostomia (dry mouth), and hypotension (due to its $\alpha_2$ activity). * **Drug of Choice for Spasticity:** Baclofen is often the first-line treatment for spasticity associated with multiple sclerosis or spinal cord injury. * **Dantrolene:** Unlike the options above, this is a **peripherally acting** muscle relaxant that inhibits calcium release from the sarcoplasmic reticulum by binding to **Ryanodine receptors (RyR1)**. It is the drug of choice for Malignant Hyperthermia.

Question 450: Which derivative of morphine is primarily used for the treatment of diarrhea?

A. Oxymorphine
B. Diphenoxylate (Correct Answer)
C. Pethidine
D. Codeine

Explanation: **Explanation:** The correct answer is **Diphenoxylate**. **Why Diphenoxylate is correct:** Diphenoxylate is a synthetic opioid derivative of pethidine, specifically designed for its **anti-diarrheal** properties. It acts primarily on the $\mu$-opioid receptors in the gastrointestinal tract, decreasing intestinal motility (stasis) and increasing the transit time of luminal contents. This allows for greater water absorption, thereby firming the stool. To prevent abuse, it is commercially formulated with a sub-therapeutic dose of **Atropine** (Lomotil), which causes unpleasant anticholinergic side effects if taken in large quantities. **Analysis of Incorrect Options:** * **Oxymorphine:** A potent semi-synthetic opioid analgesic primarily used for the relief of moderate to severe pain. It has high abuse potential and is not used for diarrhea. * **Pethidine (Meperidine):** A phenylpiperidine derivative used as an analgesic, particularly in labor and for post-operative shivering. While it causes constipation like most opioids, its systemic side effects and metabolite toxicity (normeperidine) make it unsuitable for treating diarrhea. * **Codeine:** A natural opium alkaloid used as an analgesic and a potent **antitussive** (cough suppressant). While it can cause constipation, it is not the primary clinical choice for diarrhea compared to peripherally acting agents. **High-Yield Clinical Pearls for NEET-PG:** * **Loperamide** is another pethidine derivative used for diarrhea. It is preferred over diphenoxylate because it does not cross the blood-brain barrier (no CNS effects) and has no abuse potential. * **Mechanism:** Opioids treat diarrhea by inhibiting acetylcholine release from the myenteric plexus, leading to segmental contraction and decreased longitudinal propulsive activity. * **Contraindication:** Anti-diarrheals should be avoided in infectious diarrhea (e.g., *C. difficile* or *Salmonella*) as they may delay the clearance of toxins, leading to toxic megacolon.

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