Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 81: Which of the following is a known side effect of Phenytoin toxicity?

A. Gingival hyperplasia (Correct Answer)
B. Acne rosacea
C. Exacerbation of acne vulgaris
D. Alopecia

Explanation: **Explanation:** **Phenytoin** is a widely used hydantoin derivative for the treatment of generalized tonic-clonic and focal seizures [4, 5]. It acts by blocking voltage-gated sodium channels in their inactive state [4]. **1. Why Gingival Hyperplasia is Correct:** Gingival hyperplasia (overgrowth of the gums) is a classic, dose-independent side effect occurring in approximately 20–50% of patients. The underlying mechanism involves the stimulation of **platelet-derived growth factor (PDGF)** and the proliferation of fibroblasts, leading to increased collagen deposition in the gingival connective tissue. It is often exacerbated by poor oral hygiene [1]. **2. Analysis of Incorrect Options:** * **B & C (Acne):** While Phenytoin is associated with **acneiform eruptions** or the development of **acne vulgaris**, it is not specifically linked to *Acne rosacea*. Furthermore, while it can exacerbate acne, gingival hyperplasia is a more definitive and frequently tested "signature" side effect of the drug. * **D (Alopecia):** Phenytoin does not cause hair loss; instead, it is characteristically associated with **hirsutism** (excessive body hair growth), which makes it less desirable for female patients [1]. **3. High-Yield Clinical Pearls for NEET-PG:** To remember Phenytoin’s side effects, use the mnemonic **"PH_E_N_Y_T_O_I_N"**: * **P**-450 Induction [2] * **H**-yperplasia of gums / **H**-irsutism [1] * **E**-steomalacia (due to Vitamin D interference) [1] * **N**-ystagmus (earliest sign of toxicity) [1] * **Y**-ellow-brown skin pigmentation * **T**-eratogenicity (**Fetal Hydantoin Syndrome**: cleft lip/palate, digital hypoplasia) * **O**-steomalacia [1] * **I**-nsulin inhibition (leading to hyperglycemia) * **N**-europathy (peripheral) and **Megaloblastic Anemia** (due to folate deficiency) [1]. **Note:** Phenytoin follows **Zero-order kinetics** (saturation kinetics) at therapeutic concentrations, meaning small dose increases can lead to disproportionate rises in plasma levels and toxicity [2].

Question 82: What is the drug of choice for Restless Legs Syndrome?

A. Febuxostat
B. Diazepam
C. Ropinirole (Correct Answer)
D. Baclofen

Explanation: **Explanation:** **Restless Legs Syndrome (RLS)**, also known as Willis-Ekbom Disease, is a neurological sensory-motor disorder characterized by an irresistible urge to move the legs, usually associated with unpleasant sensations that worsen at rest and during the night. **Why Ropinirole is the Correct Answer:** The pathophysiology of RLS is linked to **dopaminergic dysfunction** in the nigrostriatal pathway and iron deficiency in the CNS. Therefore, **Dopamine Agonists** are the first-line treatment (Drug of Choice). **Ropinirole** and **Pramipexole** (non-ergot dopamine agonists) are FDA-approved and preferred because they directly stimulate D2 and D3 receptors, effectively relieving symptoms and improving sleep quality. **Analysis of Incorrect Options:** * **A. Febuxostat:** A xanthine oxidase inhibitor used for the management of chronic hyperuricemia in Gout. It has no role in dopamine modulation. * **B. Diazepam:** A benzodiazepine used for anxiety and muscle spasms. While it may help with sleep onset, it does not treat the underlying sensory-motor urge of RLS and carries a risk of dependence. * **D. Baclofen:** A GABA-B agonist used as a centrally acting muscle relaxant for spasticity (e.g., Multiple Sclerosis). It is not effective for RLS. **High-Yield Clinical Pearls for NEET-PG:** 1. **First-line agents:** Ropinirole, Pramipexole, and Rotigotine (transdermal patch). 2. **Alternative/Second-line:** Gabapentin enacarbil or Pregabalin (preferred if the patient has comorbid neuropathy or insomnia). 3. **Iron Stores:** Always check **Serum Ferritin** levels. Iron supplementation is indicated if ferritin is <75 µg/L, as iron is a cofactor for tyrosine hydroxylase (the rate-limiting enzyme in dopamine synthesis). 4. **Augmentation:** A unique side effect where long-term dopamine agonist use causes symptoms to start earlier in the day or become more severe.

Question 83: What is the mechanism of action of tetanospasmin?

A. Inhibition of GABA release (Correct Answer)
B. Inhibition of cAMP
C. Inactivation of Ach receptors
D. Inhibition of cGMP

Explanation: **Mechanism of Action of Tetanospasmin** **Correct Answer: A. Inhibition of GABA release** **Explanation:** Tetanospasmin is an extremely potent exotoxin produced by *Clostridium tetani*. Its mechanism involves retrograde axonal transport from the neuromuscular junction to the cell bodies of spinal inhibitory interneurons (specifically **Renshaw cells**) [1]. The toxin acts as a zinc-dependent endopeptidase that cleaves **SNARE proteins** (specifically synaptobrevin/VAMP) [2]. This cleavage prevents the fusion of synaptic vesicles with the presynaptic membrane, thereby inhibiting the release of inhibitory neurotransmitters—**GABA** (Gamma-Aminobutyric Acid) and **Glycine** [1]. The loss of these "brakes" on the motor system leads to unopposed excitatory discharge from alpha-motor neurons, resulting in the characteristic spastic paralysis, muscle rigidity, and spasms seen in clinical tetanus [1]. **Why Incorrect Options are Wrong:** * **B & D (Inhibition of cAMP/cGMP):** These are secondary messenger systems. While toxins like *Vibrio cholerae* (cAMP) or *E. coli* ST (cGMP) affect these pathways, tetanospasmin acts directly on neurotransmitter release machinery. * **C (Inactivation of Ach receptors):** This describes the pathology of Myasthenia Gravis. Conversely, *Botulinum toxin* inhibits the release of Acetylcholine (ACh), leading to flaccid paralysis—the opposite clinical presentation of tetanus. **High-Yield Clinical Pearls for NEET-PG:** * **Target Protein:** Synaptobrevin (SNARE protein) [2]. * **Clinical Triad:** Trismus (lockjaw), Risus sardonicus (grimace), and Opisthotonus (archback) [1]. * **Comparison:** Tetanus = Spastic paralysis (Inhibits GABA/Glycine); Botulism = Flaccid paralysis (Inhibits ACh). * **Management:** Diazepam (a GABA agonist) is used to control spasms by compensating for the lost inhibitory tone.

Question 84: Increasing doses of alcohol depress brain function in which of the following orders?

A. Cerebellum, Frontal lobe, Occipital lobe
B. Frontal lobe, Cerebellum, Occipital lobe
C. Frontal lobe, Occipital lobe, Cerebellum (Correct Answer)
D. Cerebellum, Occipital lobe, Frontal lobe

Explanation: **Explanation:** Alcohol (Ethanol) is a CNS depressant that follows a **descending pattern of depression**, affecting the brain from the highest evolutionary functions (cortical) to the most primitive (medullary). The order of depression is determined by the sensitivity of specific brain regions to increasing Blood Alcohol Concentrations (BAC). 1. **Frontal Lobe (First):** This area is most sensitive. Depression leads to the loss of "inhibitory control," resulting in euphoria, talkativeness, and loss of social judgment. 2. **Occipital Lobe (Second):** As doses increase, the visual cortex is affected, leading to blurred vision, diplopia (double vision), and altered depth perception. 3. **Cerebellum (Third):** Higher concentrations affect motor coordination, resulting in the classic signs of ataxia, slurred speech, and loss of equilibrium. 4. **Medulla (Last):** At toxic levels, the vital centers (respiratory and vasomotor) are depressed, which can lead to coma and death. **Analysis of Incorrect Options:** * **Options A & D:** Incorrect because the cerebellum is affected much later than the frontal cortex. Ataxia usually occurs after the initial stage of disinhibition. * **Option B:** Incorrect because visual disturbances (Occipital) typically precede the gross motor incoordination (Cerebellar) seen in severe intoxication. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Alcohol enhances **GABA-A** receptors (increasing Cl- conductance) and inhibits **NMDA** (Glutamate) receptors. * **Metabolism:** Follows **Zero-order kinetics** (constant amount metabolized per unit time). * **Wernicke-Korsakoff Syndrome:** Caused by Thiamine (B1) deficiency; characterized by the triad of Ataxia, Ophthalmoplegia, and Confusion. * **Legal Limit:** In India, the legal limit for driving is **30 mg/100 ml** of blood.

Question 85: What is the drug of choice for nocturnal enuresis?

A. Imipramine (Correct Answer)
B. Diazepam
C. Amoxapine
D. Reboxetine

Explanation: **Explanation:** **Imipramine** is a Tricyclic Antidepressant (TCA) and is considered the drug of choice for the pharmacological management of **nocturnal enuresis** (bedwetting) in children above 6 years of age. Its efficacy is attributed to a triple mechanism of action: 1. **Anticholinergic effect:** It increases the bladder capacity by relaxing the detrusor muscle. 2. **Alpha-adrenergic stimulation:** It increases the tone of the internal urethral sphincter. 3. **Alteration of sleep architecture:** It lightens the depth of sleep, making the child more likely to wake up when the bladder is full. **Analysis of Incorrect Options:** * **Diazepam:** A benzodiazepine used for anxiety and seizures. It deepens sleep (Stage 4), which could potentially worsen enuresis. * **Amoxapine:** A tetracyclic antidepressant with dopamine-blocking properties. It is primarily used for major depression with psychotic features, not enuresis. * **Reboxetine:** A selective norepinephrine reuptake inhibitor (SNRI) used for depression; it lacks the significant anticholinergic profile required to treat enuresis effectively. **Clinical Pearls for NEET-PG:** * **First-line management:** Non-pharmacological therapy (bladder training and **enuresis alarms**) is always preferred before starting drugs. * **Alternative Drug:** **Desmopressin** (ADH analogue) is often preferred in modern practice due to a better safety profile, but in the context of classic pharmacology questions, Imipramine remains the standard answer. * **Caution:** Imipramine has a narrow therapeutic index; overdose can lead to life-threatening cardiac arrhythmias (QRS prolongation).

Question 86: Which of the following drugs is NOT used in the management of multiple sclerosis?

A. Interferon-β-1a
B. Interferon-β-1b
C. Glatiramer acetate
D. Mycophenolate (Correct Answer)

Explanation: **Explanation:** Multiple Sclerosis (MS) is a chronic autoimmune inflammatory disease of the CNS characterized by demyelination [3]. Management involves treating acute relapses (Corticosteroids) and using **Disease-Modifying Therapies (DMTs)** to reduce relapse rates. **Why Mycophenolate is the Correct Answer:** **Mycophenolate mofetil** is an immunosuppressant that inhibits inosine monophosphate dehydrogenase (IMPDH), primarily used to prevent organ transplant rejection and in certain autoimmune conditions like Lupus Nephritis [2]. While it has been studied off-label for MS, it is **not** a standard or FDA-approved DMT for Multiple Sclerosis. **Analysis of Incorrect Options:** * **Interferon-β-1a & 1b:** These are first-line injectable DMTs. They work by modulating the immune response, reducing T-cell proliferation, and decreasing the passage of inflammatory cells across the blood-brain barrier. * **Glatiramer acetate:** A synthetic polypeptide mixture that mimics **Myelin Basic Protein (MBP)**. It acts as a "decoy," diverting the autoimmune attack away from the patient's myelin [1]. It is a standard first-line therapy, especially preferred in pregnancy. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice for Acute Relapse:** Intravenous Methylprednisolone. * **Oral DMTs for MS:** Fingolimod (S1P modulator), Teriflunomide (DHODH inhibitor), and Dimethyl fumarate. * **Monoclonal Antibodies:** **Natalizumab** (targets α4β1-integrin; risk of PML) [4] and **Ocrelizumab** (targets CD20). * **Glatiramer Acetate** is generally considered the safest DMT during pregnancy.

Question 87: Which of the following is NOT a dopaminergic agonist used for the management of Parkinsonism?

A. Bromocriptine
B. Ropinirole
C. Pramipexole
D. Selegiline (Correct Answer)

Explanation: ### Explanation The correct answer is **Selegiline** because it is a **Selective irreversible MAO-B inhibitor**, not a direct dopaminergic agonist. #### 1. Why Selegiline is the Correct Answer Selegiline works by inhibiting the enzyme **Monoamine Oxidase-B (MAO-B)**, which is responsible for the degradation of dopamine in the striatum. By blocking this enzyme, it increases the availability and prolongs the action of endogenous or exogenous dopamine. It does not directly stimulate dopamine receptors. #### 2. Analysis of Incorrect Options (Dopamine Agonists) These drugs act by directly stimulating dopamine receptors (primarily D2 and D3) in the brain: * **Bromocriptine (Option A):** An older, **Ergot-derived** dopamine agonist. Its use has declined due to side effects like retroperitoneal/pulmonary fibrosis and heart valve erythromelalgia. * **Ropinirole (Option B):** A **Non-ergot** dopamine agonist. It is highly selective for D2/D3 receptors and is frequently used in early Parkinsonism and Restless Leg Syndrome. * **Pramipexole (Option C):** Another **Non-ergot** agonist with antioxidant properties. Like Ropinirole, it is preferred over ergot derivatives due to a better safety profile. #### 3. NEET-PG High-Yield Pearls * **Cheese Reaction:** Selegiline is selective for MAO-B at low doses, so it **does not** typically cause the "cheese reaction" (hypertensive crisis) associated with non-selective MAO inhibitors. However, selectivity is lost at higher doses. * **Metabolism:** Selegiline is metabolized into **amphetamine and methamphetamine**, which can cause insomnia as a side effect. * **Neuroprotection:** Selegiline is hypothesized to have neuroprotective effects by reducing oxidative stress, though clinical evidence remains debated. * **Apomorphine:** Remember that Apomorphine is a potent injectable dopamine agonist used for "rescue" therapy during "off" episodes in advanced Parkinson’s.

Question 88: Which of the following drugs increases GABA levels in the brain?

A. Diazepam
B. Phenobarbitone
C. Sodium valproate (Correct Answer)
D. Carbamazepine

Explanation: **Explanation:** The question focuses on the mechanism of action of various antiepileptic and sedative-hypnotic drugs regarding their interaction with the GABAergic system. **Why Sodium Valproate is Correct:** Sodium Valproate is a broad-spectrum antiepileptic that increases the concentration of GABA (the primary inhibitory neurotransmitter) in the brain through a pleiotropic mechanism: 1. **Inhibition of GABA Transaminase (GABA-T):** It inhibits the enzyme responsible for the degradation of GABA. 2. **Stimulation of Glutamic Acid Decarboxylase (GAD):** It enhances the enzyme that synthesizes GABA from glutamate. 3. It also blocks voltage-gated sodium channels and T-type calcium channels. **Analysis of Incorrect Options:** * **Diazepam (Benzodiazepine):** It does not increase GABA levels. Instead, it acts as a **Positive Allosteric Modulator**. It increases the **frequency** of chloride channel opening in the presence of GABA. * **Phenobarbitone (Barbiturate):** It also does not increase GABA levels. It increases the **duration** of chloride channel opening and can act as a GABA-mimetic at high doses. * **Carbamazepine:** Its primary mechanism is the **blockade of use-dependent voltage-gated sodium channels**, preventing high-frequency repetitive firing of action potentials. It has no significant effect on GABA levels. **High-Yield Clinical Pearls for NEET-PG:** * **Vigabatrin** is another drug that increases GABA levels by **irreversibly** inhibiting GABA Transaminase (associated with visual field contraction). * **Tiagabine** increases GABA levels in the synaptic cleft by inhibiting the GABA transporter (**GAT-1**), preventing reuptake. * **Valproate** is the drug of choice for Generalized Tonic-Clonic Seizures (GTCS), Myoclonic seizures, and Absence seizures, but is highly **teratogenic** (causes Neural Tube Defects).

Question 89: Tizanidine produces its action by acting on which of the following receptors?

A. Alpha adrenergic receptors (Correct Answer)
B. GABA A receptors
C. GABA B receptors
D. Beta adrenergic receptors

Explanation: **Explanation:** **Tizanidine** is a centrally acting skeletal muscle relaxant primarily used to manage spasticity associated with conditions like multiple sclerosis or spinal cord injury [2]. **1. Why Option A is Correct:** Tizanidine is a **selective Alpha-2 ($\alpha_2$) adrenergic agonist** [1]. It acts predominantly in the spinal cord. By stimulating presynaptic $\alpha_2$ receptors, it inhibits the release of excitatory amino acids (like glutamate and aspartate) from spinal interneurons [1]. This reduces the excitability of spinal motor neurons, thereby decreasing muscle spasticity without significantly affecting muscle strength. **2. Why Incorrect Options are Wrong:** * **Option B (GABA-A):** Receptors targeted by **Benzodiazepines** (e.g., Diazepam) [1]. While Diazepam is used for muscle spasms, it acts by increasing the frequency of chloride channel opening. * **Option C (GABA-B):** Receptors targeted by **Baclofen** [1]. Baclofen is a structural analog of GABA that acts pre- and post-synaptically to inhibit spinal reflexes [3]. * **Option D (Beta adrenergic):** Beta-blockers (like Propranolol) are used for essential tremors, but beta-agonists do not have a role in treating spasticity; in fact, they may cause muscle tremors as a side effect. **High-Yield Clinical Pearls for NEET-PG:** * **Side Effects:** The most common side effects of Tizanidine are **hypotension** (due to its structural similarity to Clonidine), sedation, and xerostomia (dry mouth). * **Comparison:** Unlike Baclofen, Tizanidine causes less muscle weakness, making it preferable for patients who require muscle strength for walking. * **Metabolism:** It is metabolized by **CYP1A2**; therefore, it is contraindicated with potent CYP1A2 inhibitors like Ciprofloxacin or Fluvoxamine.

Question 90: Which of the following physiological effects is NOT produced by stimulation of the Kappa opioid receptor?

A. Sedation
B. Diuresis
C. Meiosis
D. Constipation (Correct Answer)

Explanation: To master opioid pharmacology for NEET-PG, it is essential to differentiate between the three primary receptor subtypes: Mu (μ), Kappa (κ), and Delta (δ). [1] ### **Explanation of the Correct Answer** **Constipation** is primarily mediated by **Mu (μ) receptors** located in the myenteric plexus of the gastrointestinal tract. [3] Stimulation of μ-receptors decreases intestinal motility and secretions, leading to opioid-induced constipation. While Kappa receptors are present in the gut, their role in slowing transit is negligible compared to Mu receptors. Therefore, constipation is not a characteristic effect of Kappa stimulation. [2] ### **Analysis of Incorrect Options** * **Sedation (A):** Kappa receptor stimulation is well-known for producing sedation and dysphoria (unlike the euphoria associated with Mu receptors). [1] * **Diuresis (B):** This is a high-yield distinction. While Mu receptors cause urinary retention (by increasing sphincter tone), **Kappa receptors inhibit ADH (Vasopressin) release**, leading to a potent diuretic effect. * **Miosis (C):** Both Mu and Kappa receptors contribute to pupillary constriction (pinpoint pupils) via the stimulation of the Edinger-Westphal nucleus. ### **High-Yield Clinical Pearls for NEET-PG** * **Kappa (κ) Signature:** Analgesia (spinal), Sedation, Dysphoria, Psychotomimetic effects (hallucinations), and Diuresis. [1] * **Mu (μ) Signature:** Supraspinal analgesia, Euphoria, Respiratory depression, Constipation, and Physical dependence. [3] * **Pure Kappa Agonist:** Pentazocine and Butorphanol (Mixed agonist-antagonists) act primarily on Kappa for analgesia while antagonizing Mu receptors. * **Key Mnemonic:** **K**appa causes **K**icking of water out (Diuresis) and **K**ooky feelings (Dysphoria/Hallucinations).

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General Anesthetics

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Local Anesthetics

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Sedative-Hypnotics

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Antiepileptic Drugs

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Antiparkinsonian Drugs

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Opioid Analgesics

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Drugs of Abuse and Addiction

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Psychostimulants

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Hallucinogens

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CNS Stimulants and Cognitive Enhancers

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