Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 71: A 21-year-old student with schizophrenia is currently asymptomatic after two months of treatment with 3 mg of risperidone. The patient had no prior psychiatric episodes and no family history of psychiatric illness. For how long should risperidone be continued?

A. At least 6 months of symptom remission
B. At least 12 months of symptom remission (Correct Answer)
C. 2 years
D. 5 years

Explanation: **Explanation:** The management of a first-episode psychosis (FEP) requires a balance between preventing relapse and minimizing long-term side effects of antipsychotics. According to standard psychiatric guidelines (APA and NICE), for a patient with a **first episode of schizophrenia** who has achieved symptom remission, maintenance therapy should be continued for **at least 12 months**. 1. **Why Option B is Correct:** In a first-episode patient like this 21-year-old, the risk of relapse is highest in the first year following the acute phase. Continuing Risperidone for 12 months post-remission significantly reduces the risk of recurrence. If the patient remains stable after this period, a gradual tapering of the dose can be considered. 2. **Why Options A, C, and D are Incorrect:** * **6 months (Option A):** This duration is generally considered too short for schizophrenia; it is more characteristic of the treatment duration for a single episode of Major Depressive Disorder. * **2 years (Option C):** This is often recommended for patients who have had **multiple episodes** (recurrent schizophrenia) but are not yet considered "chronic." * **5 years (Option D):** This prolonged duration is reserved for patients with a history of frequent relapses, violent behavior, or severe functional impairment. **High-Yield Clinical Pearls for NEET-PG:** * **First Episode:** Treat for 1–2 years post-remission. * **Multiple Episodes:** Treat for 2–5 years. * **Chronic/Relapsing:** May require lifelong therapy. * **Risperidone Fact:** It is a second-generation antipsychotic (SGA) that blocks both $D_2$ and $5-HT_{2A}$ receptors. At doses >6 mg, it carries a higher risk of Extrapyramidal Symptoms (EPS) and Hyperprolactinemia compared to other SGAs.

Question 72: What is the mechanism of action of Memantine?

A. GABAergic action
B. Glutaminergic action (Correct Answer)
C. Adrenergic action
D. Dopaminergic action

Explanation: **Explanation:** **1. Why Option B is Correct:** Memantine is a drug primarily used in the management of moderate-to-severe Alzheimer’s disease. Its mechanism of action is as a **low-affinity, non-competitive NMDA (N-methyl-D-aspartate) receptor antagonist**. NMDA receptors are a subtype of **glutamate** receptors. In Alzheimer’s, overstimulation of these receptors by glutamate leads to excessive calcium influx, causing "excitotoxicity" and neuronal death. Memantine blocks these receptors under conditions of excessive stimulation, thereby preventing neurodegeneration while allowing normal physiological signaling to occur. **2. Why Other Options are Incorrect:** * **Option A (GABAergic):** GABA is the primary inhibitory neurotransmitter. Drugs like benzodiazepines or barbiturates act here, but they are not used to treat the cognitive decline in Alzheimer's. * **Option C (Adrenergic):** Adrenergic drugs affect the sympathetic nervous system (alpha/beta receptors). They have no direct role in the primary pathology of Alzheimer’s dementia. * **Option D (Dopaminergic):** Dopaminergic agents (like Levodopa) are used in Parkinson’s disease. While some dementia patients have motor symptoms, Memantine does not act on dopamine receptors. **3. High-Yield Clinical Pearls for NEET-PG:** * **Indication:** Specifically approved for **moderate-to-severe** Alzheimer’s (unlike Donepezil, which is used for all stages). * **Combination Therapy:** Often used in combination with Cholinesterase inhibitors (e.g., Donepezil) for synergistic effects. * **Side Effects:** Generally well-tolerated; the most common side effects are dizziness, headache, and confusion. * **Neuroprotection:** It is considered neuroprotective because it prevents glutamate-induced excitotoxicity.

Question 73: Which of the following is a long-acting dopamine agonist?

A. Bromocriptine
B. Lisuride
C. Cabergoline (Correct Answer)
D. Apomorphine

Explanation: **Explanation:** **Cabergoline** is the correct answer because it is a potent, synthetic ergoline derivative with a very high affinity for **D2 receptors** and an exceptionally long half-life (approximately **65–110 hours**). This long duration of action allows for convenient **once or twice-weekly dosing**, making it the preferred agent for treating hyperprolactinemia and prolactinomas. **Analysis of Incorrect Options:** * **Bromocriptine (Option A):** An older ergoline derivative and D2 agonist. It has a short half-life (about 2–8 hours) and requires daily dosing (usually 2–3 times a day). It is more commonly associated with GI side effects compared to Cabergoline. * **Lisuride (Option B):** An ergoline derivative that acts as a dopamine agonist. It has a very short half-life (approx. 2 hours) and is rarely used clinically now due to the availability of superior long-acting agents. * **Apomorphine (Option C):** A non-ergoline dopamine agonist with a very rapid onset but a very short duration of action (half-life ~40 minutes). It is used as a "rescue" medication via subcutaneous injection for "off" episodes in advanced Parkinson’s disease. **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Cabergoline is the DOC for **Prolactinoma** due to better efficacy and tolerability than Bromocriptine. * **Ergot vs. Non-Ergot:** Bromocriptine and Cabergoline are **Ergot derivatives** (risk of pulmonary/cardiac fibrosis). Pramipexole and Ropinirole are **Non-ergot derivatives** (preferred in Parkinson’s to avoid fibrosis). * **Side Effects:** Dopamine agonists can cause impulse control disorders (pathological gambling, hypersexuality) and sudden sleep attacks.

Question 74: Botulinum toxin acts on which of the following?

A. Inhibition of neurotransmitter release (Correct Answer)
B. Post-synaptic blockade
C. At the synapse
D. Reuptake inhibition

Explanation: **Explanation:** **Mechanism of Action (Why A is correct):** Botulinum toxin (produced by *Clostridium botulinum*) is a potent neurotoxin that acts pre-synaptically. It functions as a zinc-dependent endopeptidase that cleaves **SNARE proteins** (specifically Synaptobrevin, SNAP-25, and Syntaxin). These proteins are essential for the docking and fusion of acetylcholine-containing vesicles with the neuronal membrane. By destroying these proteins, the toxin prevents the exocytosis of acetylcholine into the synaptic cleft, leading to **inhibition of neurotransmitter release**. This results in flaccid paralysis. **Analysis of Incorrect Options:** * **B. Post-synaptic blockade:** This is the mechanism of drugs like d-Tubocurarine (competitive NMJ blockers) or Bungarotoxin, which bind to nicotinic receptors. Botulinum toxin acts pre-synaptically, not on the receptors. * **C. At the synapse:** While the effect is observed at the synapse, this option is too vague. Pharmacology exams require the specific functional site (pre-synaptic vs. post-synaptic). * **D. Reuptake inhibition:** This is the mechanism of drugs like Cocaine or SSRIs, which increase neurotransmitter concentration in the cleft. Botulinum toxin decreases the concentration by preventing release. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Causes "Floppy Infant Syndrome" (ingestion of honey containing spores) and foodborne botulism (ingestion of preformed toxin in canned food). * **Therapeutic Uses:** Used for focal dystonias (Blepharospasm, Spasmodic torticollis), Achalasia cardia, Hyperhidrosis, and cosmetic reduction of wrinkles (Botox). * **Contrast with Tetanus Toxin:** While both cleave SNARE proteins, Tetanus toxin undergoes retrograde axonal transport to act on inhibitory interneurons (Renshaw cells) in the spinal cord, causing spastic paralysis.

Question 75: Which anti-epileptic drug is known to cause hyponatremia?

A. Magnesium valproate
B. Phenytoin
C. Carbamazepine (Correct Answer)
D. Ethosuximide

Explanation: **Explanation:** **Carbamazepine (Option C)** is the correct answer because it is well-known for causing **hyponatremia** through a mechanism similar to the Syndrome of Inappropriate Antidiuretic Hormone (SIADH). It increases the sensitivity of the renal tubules to ADH and can also stimulate the direct release of ADH from the posterior pituitary. This leads to water retention and dilutional hyponatremia. This side effect is more common in elderly patients and those taking concurrent diuretics. **Analysis of Incorrect Options:** * **Magnesium valproate (A):** Valproate is primarily associated with hepatotoxicity, weight gain, alopecia, and pancreatitis. It does not typically cause hyponatremia. * **Phenytoin (B):** Phenytoin is notorious for gingival hyperplasia, hirsutism, and megaloblastic anemia (due to folate deficiency). Interestingly, it can actually *inhibit* ADH secretion, occasionally leading to the opposite effect (diabetes insipidus-like state). * **Ethosuximide (D):** This is the drug of choice for Absence seizures. Its primary side effects are GI distress, lethargy, and Stevens-Johnson Syndrome, but not electrolyte imbalances like hyponatremia. **High-Yield Clinical Pearls for NEET-PG:** * **Oxcarbazepine**, a structural derivative of carbamazepine, carries an even **higher risk** of hyponatremia than carbamazepine itself. * Carbamazepine is a potent **enzyme inducer** (CYP3A4) and exhibits **auto-induction** (it induces its own metabolism). * It is the **Drug of Choice (DOC)** for Trigeminal Neuralgia. * Always screen for the **HLA-B*1502** allele in patients of Asian descent before starting Carbamazepine to prevent life-threatening Stevens-Johnson Syndrome (SJS).

Question 76: Benzodiazepines differ from barbiturates in the following aspects except:

A. They have a steeper dose-response curve (Correct Answer)
B. They have a higher therapeutic index
C. They have lower abuse liability
D. They do not induce microsomal drug-metabolizing enzymes

Explanation: The core difference between Benzodiazepines (BZDs) and Barbiturates lies in their mechanism of action on the GABA-A receptor and their subsequent safety profiles. **Why Option A is the Correct Answer:** Benzodiazepines actually have a **flatter (shallower) dose-response curve** compared to barbiturates. This is because BZDs are **GABA-facilitatory** (they increase the *frequency* of chloride channel opening) and require endogenous GABA to work. Once GABA receptors are saturated, further increases in BZD dose do not result in further CNS depression. In contrast, Barbiturates have a **steeper dose-response curve** because they are **GABA-mimetic** at high doses (they increase the *duration* of channel opening even in the absence of GABA), leading to progressive CNS depression, coma, and death [1]. **Analysis of Incorrect Options:** * **Option B:** BZDs have a **higher therapeutic index** because of the ceiling effect mentioned above, making them much safer in overdose than barbiturates [2]. * **Option C:** BZDs have **lower abuse liability** and milder withdrawal symptoms compared to the severe physical dependence associated with barbiturates. * **Option D:** BZDs **do not induce** hepatic microsomal enzymes (CYP450), whereas barbiturates are potent enzyme inducers, leading to numerous drug-drug interactions (e.g., with warfarin or oral contraceptives). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism Mnemonic:** **B**enzodiazepines increase **F**requency (**B-F**); **B**arbiturates increase **D**uration (**B-D**). * **Antidote:** Flumazenil is a specific antagonist for BZD overdose; there is no specific antagonist for barbiturates [1]. * **Contraindication:** Barbiturates are strictly contraindicated in **Acute Intermittent Porphyria** as they induce ALA synthase [1].

Question 77: Which is the central inhibitor of dopamine metabolism?

A. Carbidopa
B. Deperenyl (Correct Answer)
C. Orphenadrine
D. Bromocriptine

Explanation: **Explanation:** The correct answer is **Deperenyl** (also known as **Selegiline**). **1. Why Deperenyl is correct:** Dopamine metabolism in the brain occurs primarily via two enzymes: Monoamine Oxidase-B (MAO-B) and Catechol-O-Methyltransferase (COMT). Deperenyl is a selective, irreversible inhibitor of **MAO-B**. By inhibiting this enzyme centrally, it prevents the breakdown of dopamine in the striatum, thereby increasing its bioavailability and prolonging its action. This makes it a key "dopamine sparer" in the management of Parkinson’s disease. **2. Why the other options are incorrect:** * **Carbidopa:** It is a **peripheral** inhibitor of Dopa-decarboxylase. It prevents the conversion of Levodopa to dopamine in the systemic circulation but does not cross the blood-brain barrier (BBB). It does not inhibit dopamine metabolism centrally. * **Orphenadrine:** This is a central **anticholinergic** drug used to manage the cholinergic overactivity (tremors/rigidity) seen in Parkinsonism. It has no direct effect on dopamine metabolism. * **Bromocriptine:** This is a **Dopamine Agonist** (specifically at D2 receptors). It mimics the action of dopamine rather than inhibiting its metabolic breakdown. **3. Clinical Pearls for NEET-PG:** * **MAO Inhibitors:** Selegiline and Rasagiline are selective MAO-B inhibitors. Unlike non-selective MAO inhibitors, they do not typically cause the "Cheese Reaction" (hypertensive crisis) at conventional doses. * **COMT Inhibitors:** **Tolcapone** acts both centrally and peripherally, while **Entacapone** acts only peripherally. * **Metabolic End-product:** The major central metabolite of dopamine is **Homovanillic Acid (HVA)**. * **Drug of Choice:** For drug-induced Parkinsonism, central anticholinergics like **Benztropine** or **Trihexyphenidyl** are preferred over levodopa.

Question 78: Which of the following drugs is not an opioid agonist?

A. Heroin
B. Ketamine (Correct Answer)
C. Methadone
D. Codeine

Explanation: **Explanation:** The correct answer is **Ketamine** because it is a **non-competitive NMDA (N-methyl-D-aspartate) receptor antagonist**, not an opioid agonist. While ketamine provides significant analgesia, its mechanism involves blocking glutamate receptors in the brain and spinal cord, leading to "dissociative anesthesia." **Analysis of Options:** * **Heroin (Diacetylmorphine):** A highly lipid-soluble semi-synthetic **opioid agonist**. It is rapidly converted to morphine in the brain and acts primarily on $\mu$-opioid receptors. * **Methadone:** A synthetic **long-acting $\mu$-opioid agonist**. It is clinically significant for its use in opioid detoxification and maintenance programs due to its long half-life and minimal withdrawal symptoms. * **Codeine (3-methylmorphine):** A natural opium alkaloid that acts as a **weak opioid agonist**. It is a prodrug converted to morphine by the enzyme CYP2D6 and is commonly used as an antitussive and for mild-to-moderate pain. **High-Yield Clinical Pearls for NEET-PG:** * **Ketamine Triple Effect:** It causes analgesia, amnesia, and sedation while maintaining airway reflexes and stimulating the sympathetic nervous system (increases HR and BP). * **Dissociative Anesthesia:** Characterized by a state where the patient appears awake (eyes open) but is unconscious and does not respond to sensory input. * **Emergence Delirium:** A common side effect of ketamine, which can be prevented by pre-administering **Benzodiazepines** (e.g., Midazolam). * **Drug of Choice:** Ketamine is preferred for induction in patients with **hypovolemic shock** or **bronchial asthma**.

Question 79: Which condition is treated with a transdermal patch that delivers a dopamine agonist?

A. Alzheimer's disease
B. Amyotrophic lateral sclerosis
C. Multiple myeloma
D. Parkinson's disease (Correct Answer)

Explanation: **Explanation:** The correct answer is **Parkinson’s disease**. The transdermal patch mentioned is **Rotigotine**, a non-ergot dopamine agonist (D2, D3, and D1 receptor stimulator). **1. Why Parkinson’s Disease is Correct:** Parkinson’s disease is characterized by the progressive loss of dopaminergic neurons in the substantia nigra. Treatment aims to restore dopamine levels or stimulate dopamine receptors. The **Rotigotine transdermal patch** provides continuous drug delivery over 24 hours, ensuring stable plasma concentrations. This "continuous dopaminergic stimulation" helps minimize "off" periods and motor fluctuations compared to pulsatile oral dosing. **2. Why the Other Options are Incorrect:** * **Alzheimer’s Disease:** Treated with acetylcholinesterase inhibitors (e.g., Donepezil, Rivastigmine) or NMDA antagonists (Memantine). While **Rivastigmine** is available as a patch, it is not a dopamine agonist. * **Amyotrophic Lateral Sclerosis (ALS):** Managed with **Riluzole** (glutamate antagonist) or **Edaravone** (antioxidant) to slow progression; dopamine agonists have no role here. * **Multiple Myeloma:** A hematologic malignancy treated with chemotherapy, proteasome inhibitors (Bortezomib), or immunomodulators (Thalidomide), not CNS-acting dopamine agonists. **High-Yield NEET-PG Pearls:** * **Rotigotine:** The only dopamine agonist currently delivered via a transdermal patch. * **Other Dopamine Agonists:** Pramipexole and Ropinirole (Oral); Apomorphine (Subcutaneous rescue therapy for "off" episodes). * **Side Effects:** Dopamine agonists are associated with **impulse control disorders** (pathological gambling, hypersexuality) and sudden sleep attacks. * **Rivastigmine Patch:** Often confused with Rotigotine; remember Rivastigmine is for Alzheimer’s (Cholinergic) and Rotigotine is for Parkinson’s (Dopaminergic).

Question 80: Which of the following statements regarding the use of antiepileptic drugs in pregnancy is true?

A. Valproate is associated with neural tube defects.
B. Multiple drugs should be given. (Correct Answer)
C. Carbamazepine is used as monotherapy.
D. Phenytoin can produce foetal hydantoin syndrome.

Explanation: ### Explanation The management of epilepsy during pregnancy requires a delicate balance between controlling maternal seizures and minimizing teratogenic risks to the fetus [2]. **Why Option B is Correct:** While monotherapy is the gold standard in pregnancy to minimize teratogenicity, the question asks for a "true statement" regarding the general principles of management. In clinical practice, if a patient’s seizures are not controlled by a single drug, **multiple drugs (polytherapy)** may be necessary [1]. Uncontrolled tonic-clonic seizures pose a greater risk to both the mother and fetus (due to hypoxia and trauma) than the potential risks of polytherapy [1]. Therefore, the priority is achieving seizure freedom, even if it requires a combination of drugs [1], [4]. **Analysis of Other Options:** * **Option A:** While Valproate *is* associated with neural tube defects (and is the most teratogenic AED), this is a specific side effect [2], [3]. In the context of "best practices" or "true statements" for management, the focus is often on the clinical strategy (seizure control). * **Option C:** Carbamazepine can be used as monotherapy, but it is not the *only* drug used, nor is it the preferred first-line agent in modern practice (Levetiracetam and Lamotrigine are preferred) [3]. * **Option D:** Phenytoin is indeed associated with Fetal Hydantoin Syndrome (cleft lip/palate, digital hypoplasia), but like Option A, this is a specific adverse effect rather than a guiding principle of therapy. **NEET-PG High-Yield Pearls:** * **Most Teratogenic AED:** Valproate (causes Neural Tube Defects; risk is dose-dependent) [3]. * **Safest AEDs in Pregnancy:** Levetiracetam and Lamotrigine. * **Folic Acid Supplementation:** 5 mg/day should be started pre-conceptionally for all women on AEDs to reduce the risk of NTDs. * **Vitamin K:** Administered to the neonate at birth if the mother is on enzyme-inducing drugs (Phenytoin, Phenobarbital) to prevent hemorrhagic disease of the newborn [2].

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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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