Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 411: What are the effects of long-term levodopa therapy?

A. Facial tics
B. Nightmares
C. End of dose effect
D. All the above (Correct Answer)

Explanation: **Explanation:** Levodopa remains the gold standard for Parkinson’s disease, but its long-term use (typically after 3–5 years) is limited by significant motor and psychiatric complications. The correct answer is **D (All the above)** because long-term therapy leads to both peripheral dopaminergic effects and central fluctuations. 1. **Facial Tics (Dyskinesias):** These are involuntary movements (chorea, tics, or dystonia) occurring at the peak of the drug's plasma concentration (**Peak-dose dyskinesia**). They result from the overstimulation of dopamine receptors in the striatum as the therapeutic window narrows over time. 2. **Nightmares (Psychiatric Symptoms):** Chronic levodopa therapy increases dopamine levels in the mesolimbic pathway. This can manifest as vivid dreams, nightmares, hallucinations, and even psychosis. 3. **End-of-dose effect (Wearing-off):** As the disease progresses, the nigrostriatal neurons lose their ability to store dopamine. The duration of the drug’s benefit shortens, and symptoms return before the next dose is due. **Clinical Pearls for NEET-PG:** * **On-Off Phenomenon:** A more advanced complication where the patient fluctuates unpredictably between mobility ("on") and severe rigidity ("off"), unrelated to dose timing. * **Management of "Wearing-off":** Managed by increasing the frequency of doses, adding a COMT inhibitor (Entacapone), or a MAO-B inhibitor (Selegiline). * **Management of Dyskinesias:** Often treated by adding **Amantadine** (an NMDA antagonist). * **Drug Holiday:** Historically used to restore receptor sensitivity, but now largely abandoned due to the risk of Neuroleptic Malignant-like Syndrome.

Question 412: An acute syndrome of abnormal involuntary movements resembling tardive dyskinesia is associated with excessive dosage of which of the following medications?

A. Reserpine (Correct Answer)
B. Propranolol
C. Clonidine
D. Levodopa

Explanation: **Explanation:** The correct answer is **Reserpine**. **Mechanism and Rationale:** Reserpine is an alkaloid that irreversibly blocks the **Vesicular Monoamine Transporter (VMAT-2)**. This prevents the packaging of dopamine, norepinephrine, and serotonin into synaptic vesicles, leading to the depletion of these neurotransmitters. In the nigrostriatal pathway, the profound depletion of dopamine creates a state of "chemical denervation." This can result in **extrapyramidal side effects (EPS)**, including drug-induced Parkinsonism and an acute syndrome of choreoathetotic movements that clinically mimic **Tardive Dyskinesia**. While classic tardive dyskinesia is usually a delayed effect of long-term dopamine receptor blockade (antipsychotics), Reserpine can induce these abnormal movements acutely due to rapid dopamine depletion. **Analysis of Incorrect Options:** * **B. Propranolol:** A non-selective beta-blocker used for tremors and prophylaxis of migraine; it does not affect dopaminergic transmission and is actually used to *treat* akathisia. * **C. Clonidine:** An alpha-2 agonist used in hypertension and ADHD; it reduces sympathetic outflow but does not cause dyskinesias. * **D. Levodopa:** A dopamine precursor used in Parkinson’s disease. While it causes dyskinesias, these are typically "peak-dose dyskinesias" occurring due to *excess* dopamine, not depletion, and the clinical context differs from the Reserpine-induced syndrome. **NEET-PG High-Yield Pearls:** * **Reserpine & Depression:** Due to the depletion of serotonin and NE, Reserpine is notorious for causing severe **psychotic depression** (contraindicated in patients with a history of depression). * **VMAT Inhibition:** Unlike Reserpine, **Tetrabenazine** (a reversible VMAT-2 inhibitor) is actually used to *treat* Huntington’s chorea and Tardive Dyskinesia. * **Reserpine Side Effects:** Remember the "D's": Depletion of amines, Depression, Diarrhea (due to parasympathetic dominance), and Dyskinesia.

Question 413: Which drug is used to prevent alcohol withdrawal syndrome?

A. Diazepam (Correct Answer)
B. Clonidine
C. Propranolol
D. Methadone

Explanation: **Explanation:** **1. Why Diazepam is the Correct Answer:** Benzodiazepines (BZDs) like **Diazepam** are the **gold standard and first-line treatment** for preventing and managing Alcohol Withdrawal Syndrome (AWS). Chronic alcohol consumption causes down-regulation of GABA receptors and up-regulation of NMDA receptors. Sudden cessation leads to a state of CNS hyperexcitability. Diazepam, a long-acting BZD, acts as a GABA-A agonist, providing **cross-tolerance** with alcohol. It effectively prevents seizures, delirium tremens, and stabilizes vital signs due to its long half-life and active metabolites. **2. Why Other Options are Incorrect:** * **Clonidine (B):** An alpha-2 agonist that helps reduce autonomic overactivity (tachycardia, hypertension) but **does not prevent seizures or delirium**, which are the most dangerous complications of withdrawal. * **Propranolol (C):** A beta-blocker used symptomatically to control tremors and tachycardia. Like clonidine, it lacks anti-epileptic properties and can mask the early signs of withdrawal. * **Methadone (D):** A mu-opioid receptor agonist used specifically for **Opioid withdrawal** and maintenance therapy, not for alcohol withdrawal. **3. High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Chlordiazepoxide or Diazepam are preferred due to their long half-life (self-tapering effect). * **Liver Failure Exception:** In patients with cirrhosis or liver failure, use **LOT** drugs (**L**orazepam, **O**xazepam, **T**emazepam) as they do not require hepatic oxidation and have no active metabolites. * **Wernicke’s Encephalopathy:** Always administer **Thiamine (B1)** before glucose to prevent precipitating Wernicke’s. * **Disulfiram:** Used for aversion therapy (maintenance of abstinence), never for acute withdrawal.

Question 414: Which drug does not act on the neuromuscular junction?

A. Baclofen
B. Carisoprodol
C. Haloperidol
D. All of the above (Correct Answer)

Explanation: **Explanation:** The core concept here is the distinction between **Centrally Acting Muscle Relaxants** and **Neuromuscular Blocking Agents (NMBAs)**. Drugs that act on the **Neuromuscular Junction (NMJ)** (e.g., Succinylcholine, Vecuronium) interfere with nicotinic acetylcholine receptors at the motor endplate. In contrast, the drugs listed in the options act within the Central Nervous System (CNS). * **Baclofen:** It is a GABA-B receptor agonist. It acts primarily at the spinal cord level to inhibit monosynaptic and polysynaptic reflexes. It is used for spasticity in conditions like Multiple Sclerosis or spinal cord injuries. * **Carisoprodol:** This is a sedative-centrally acting muscle relaxant. It works by altering interneuronal activity in the spinal cord and the descending reticular formation of the brain. It is metabolized to meprobamate. * **Haloperidol:** This is a typical antipsychotic that acts as a D2 receptor antagonist in the mesolimbic and nigrostriatal pathways. It has no pharmacological action at the NMJ. Since none of these drugs target the motor endplate or NMJ, **Option D (All of the above)** is correct. **High-Yield Clinical Pearls for NEET-PG:** * **Baclofen:** Drug of choice for spasticity; sudden withdrawal can cause seizures and hallucinations. * **Dantrolene:** The only peripheral muscle relaxant that does *not* act on the NMJ; it acts on **Ryanodine receptors (RyR1)** to inhibit calcium release from the sarcoplasmic reticulum. It is the DOC for **Malignant Hyperthermia**. * **Botulinum Toxin:** Acts pre-synaptically at the NMJ to prevent the release of Acetylcholine by degrading SNAP-25 proteins.

Question 415: Which of the following is not a side effect of phenytoin?

A. Hypoglycemia (Correct Answer)
B. Osteomalacia
C. Gum hypertrophy
D. Lymphadenopathy

Explanation: **Phenytoin** is a widely used hydantoin derivative for tonic-clonic and partial seizures. It has a narrow therapeutic index and a distinct side-effect profile that is frequently tested in NEET-PG [1], [2]. ### **Why Hypoglycemia is the Correct Answer** Phenytoin actually causes **Hyperglycemia**, not hypoglycemia. It inhibits the release of insulin from pancreatic beta cells. By decreasing insulin secretion, it leads to elevated blood glucose levels. Therefore, hypoglycemia is not a side effect of phenytoin. ### **Explanation of Incorrect Options** * **Osteomalacia (Option B):** Phenytoin induces hepatic microsomal enzymes (CYP450), which increases the metabolism of Vitamin D. This leads to Vitamin D deficiency, hypocalcemia, and subsequent osteomalacia in adults or rickets in children [1]. * **Gum Hypertrophy (Option C):** This is one of the most common side effects (occurring in ~20% of patients). It is due to the overgrowth of gingival tissue caused by the stimulation of alveolar fibroblasts and increased expression of platelet-derived growth factor (PDGF) [1]. * **Lymphadenopathy (Option D):** Phenytoin can cause a "Pseudolymphoma" syndrome, characterized by lymph node enlargement. In rare cases, it can progress to actual malignant lymphoma. ### **High-Yield Clinical Pearls for NEET-PG** To remember Phenytoin side effects, use the mnemonic **"HOT MALAI"**: * **H** - Hirsutism, Hypertrophy of gums, Hyperglycemia [1] * **O** - Osteomalacia [1] * **T** - Teratogenicity (Fetal Hydantoin Syndrome: cleft lip/palate, digital hypoplasia) * **M** - Megaloblastic anemia (due to decreased folate absorption) [1] * **A** - Ataxia and Nystagmus (signs of cerebellar toxicity) [1] * **L** - Lymphadenopathy * **A** - Arrhythmias (when given rapidly IV) [3] * **I** - Insulin inhibition (leading to Hyperglycemia) **Note:** Phenytoin follows **Zero-order kinetics** (saturation kinetics) at therapeutic concentrations, meaning a small dose increase can lead to a disproportionate rise in plasma levels and toxicity [2].

Question 416: A teenager presents with persistent and severe headaches. The physician diagnoses migraine and prescribes sumatriptan. By which of the following mechanisms does sumatriptan act?

A. Dopamine 1 agonist
B. GABAB antagonist
C. Muscarinic 3 antagonist
D. Serotonin 1D agonist (Correct Answer)

Explanation: **Explanation:** **Mechanism of Action (Correct Option):** Sumatriptan is a selective **5-HT$_{1B/1D}$ receptor agonist**. It is the prototype of the "Triptan" class used for the acute management of migraine. Its therapeutic effect is mediated via three primary mechanisms: 1. **Vasoconstriction:** Activation of 5-HT$_{1B}$ receptors on intracranial blood vessels leads to the constriction of painfully dilated meningeal vessels. 2. **Inhibition of Neuropeptides:** Activation of 5-HT$_{1D}$ receptors on trigeminal nerve terminals inhibits the release of pro-inflammatory neuropeptides (e.g., Calcitonin Gene-Related Peptide or CGRP and Substance P). 3. **Central Inhibition:** It reduces the transmission of pain signals through the trigeminal nucleus caudalis. **Analysis of Incorrect Options:** * **Option A (D1 Agonist):** Dopamine agonists (like Fenoldopam) are used in hypertensive emergencies or Parkinson’s disease. In migraine, dopamine *antagonists* (e.g., Metoclopramide) are used as adjuncts to treat nausea and enhance gastric emptying. * **Option B (GABA$_B$ Antagonist):** GABA$_B$ agonists (like Baclofen) are muscle relaxants. Antagonists have no established role in migraine therapy. * **Option C (M3 Antagonist):** These drugs (e.g., Oxybutynin) are used for overactive bladder or COPD. Anticholinergics are not indicated for migraine. **Clinical Pearls for NEET-PG:** * **Drug of Choice:** Triptans are the DOC for **acute severe migraine** attacks but are *not* used for prophylaxis. * **Contraindications:** Due to their vasoconstrictive properties, they are strictly contraindicated in patients with **Ischemic Heart Disease (Prinzmetal angina/MI)**, uncontrolled hypertension, and peripheral vascular disease. * **Serotonin Syndrome:** Caution is required when co-administered with SSRIs or MAO inhibitors.

Question 417: Which of the following drugs is NOT used in the management of Alzheimer's disease?

A. Rivastigmine
B. Memantine
C. Donepezil
D. Clonidine (Correct Answer)

Explanation: **Explanation:** The management of Alzheimer’s Disease (AD) primarily focuses on enhancing cholinergic transmission or modulating glutamatergic excitability. **Clonidine** is the correct answer because it is a **centrally acting alpha-2 ($\alpha_2$) adrenergic agonist** used primarily for hypertension, ADHD, and opioid withdrawal; it has no role in treating the cognitive decline of Alzheimer’s. **Analysis of Options:** * **Rivastigmine & Donepezil (Options A & C):** These are **Centrally acting Reversible Acetylcholinesterase Inhibitors (AChEIs)**. Alzheimer’s is characterized by a deficiency of acetylcholine in the nucleus basalis of Meynert. These drugs increase synaptic acetylcholine levels, improving cognitive function in mild-to-moderate AD. *Note: Rivastigmine is unique as it inhibits both AChE and Butyrylcholinesterase.* * **Memantine (Option B):** This is an **NMDA receptor antagonist**. It prevents glutamate-induced excitotoxicity, which leads to neuronal death in AD. It is typically used for moderate-to-severe cases, often in combination with Donepezil. **High-Yield Clinical Pearls for NEET-PG:** 1. **Donepezil** is the most commonly used AChEI due to its once-daily dosing and better tolerability. 2. **Galantamine** is another AChEI used in AD that also acts as a nicotinic receptor modulator. 3. **Side Effects of AChEIs:** Primarily GI-related (nausea, diarrhea) and bradycardia (SLUDGE effects). 4. **Newer Drugs:** **Aducanumab** and **Lecanemab** (monoclonal antibodies against amyloid-beta plaques) are recent FDA-approved disease-modifying therapies. 5. **Clonidine Side Effects:** Sedation, dry mouth (xerostomia), and rebound hypertension if stopped abruptly.

Question 418: Which Serotonin-Norepinephrine Reuptake Inhibitor (SNRI) is used to treat pain associated with diabetic neuropathy?

A. Clomipramine
B. Venlafaxine
C. Duloxetine (Correct Answer)
D. Desvenlafaxine

Explanation: **Explanation:** **Duloxetine** is a potent Serotonin-Norepinephrine Reuptake Inhibitor (SNRI) that is FDA-approved for the management of chronic pain conditions, specifically **diabetic peripheral neuropathic pain (DPNP)**, fibromyalgia, and chronic musculoskeletal pain. Its efficacy stems from its ability to enhance descending inhibitory pain pathways in the spinal cord by increasing the synaptic concentration of both serotonin and norepinephrine. **Analysis of Options:** * **Duloxetine (Correct):** It is the first-line SNRI for diabetic neuropathy. It has a balanced inhibition of both reuptake transporters and is preferred due to extensive clinical evidence for neuropathic pain. * **Clomipramine:** This is a Tricyclic Antidepressant (TCA). While TCAs are used off-label for neuropathy, Clomipramine is primarily used for Obsessive-Compulsive Disorder (OCD) and is not an SNRI. * **Venlafaxine:** Although it is an SNRI used off-label for neuropathic pain, it is not the primary choice or the specific answer sought for "approved" indications in this context. At lower doses, it acts primarily as an SSRI; its norepinephrine effect only becomes significant at higher doses. * **Desvenlafaxine:** The active metabolite of venlafaxine. While it is an SNRI, it is primarily indicated for Major Depressive Disorder and lacks the robust clinical labeling for diabetic neuropathy that duloxetine possesses. **High-Yield Pearls for NEET-PG:** * **First-line drugs for Diabetic Neuropathy:** Duloxetine (SNRI), Pregabalin/Gabapentin (Calcium channel α2-δ ligands), and Amitriptyline (TCA). * **Duloxetine Side Effects:** Nausea (most common), dry mouth, and somnolence. It should be avoided in patients with chronic liver disease. * **Mechanism:** SNRIs treat pain by increasing norepinephrine in the **descending antinociceptive pathway**, which inhibits the transmission of pain signals from the periphery to the brain.

Question 419: Which of the following anti-Parkinson drugs can lead to the adverse effect described?

A. Ropinirole
B. Tolcapone
C. Selegiline
D. Amantadine (Correct Answer)

Explanation: ***Amantadine*** - Causes **livedo reticularis** (net-like, mottled purplish skin discoloration) due to its **anticholinergic properties** and effects on peripheral blood flow. - This distinctive **dermatological adverse effect** occurs in up to 90% of patients on long-term amantadine therapy and is reversible upon discontinuation. *Ropinirole* - A **dopamine agonist** that primarily causes **nausea, orthostatic hypotension**, and **impulse control disorders** like gambling. - Does not cause **livedo reticularis** as it lacks the anticholinergic and vascular effects seen with amantadine. *Tolcapone* - A **COMT inhibitor** that can cause severe **hepatotoxicity** requiring regular liver function monitoring. - Main adverse effects include **dyskinesia, nausea**, and **orange discoloration of urine**, not skin mottling. *Selegiline* - A **MAO-B inhibitor** that can cause **insomnia, orthostatic hypotension**, and **serotonin syndrome** when combined with certain antidepressants. - Does not affect **peripheral circulation** or cause the characteristic skin changes associated with amantadine.

Question 420: Which is the most important drug for treating psychosis?

A. Thiopentone
B. Thioridazine (Correct Answer)
C. Theophylline
D. Thiocolchicoside

Explanation: **Thioridazine** is the correct answer because it is a **typical (first-generation) antipsychotic** belonging to the Phenothiazine class [3]. It works primarily by blocking **D2 receptors** in the mesolimbic pathway of the brain, thereby reducing the positive symptoms of psychosis such as hallucinations and delusions [2, 1]. **Analysis of Incorrect Options:** * **Thiopentone (A):** An ultra-short-acting barbiturate used primarily for the **induction of anesthesia** and in the management of refractory status epilepticus. It has no antipsychotic properties. * **Theophylline (C):** A methylxanthine derivative used as a **bronchodilator** in the treatment of asthma and COPD. It acts by inhibiting phosphodiesterase (PDE), increasing cAMP levels. * **Thiocolchicoside (D):** A muscle relaxant with anti-inflammatory and analgesic properties. It acts as a **GABA-A receptor antagonist** and is used for muscle spasms; it is contraindicated in patients with a history of epilepsy. **High-Yield NEET-PG Pearls:** * **Side Effect Profile:** Thioridazine is notorious for causing **retinal pigmentation** (pigmentary retinopathy) if used in high doses (>800 mg/day) [3]. * **Cardiac Warning:** It is the antipsychotic most commonly associated with **QT interval prolongation** and Torsades de Pointes [3]. * **Mnemonic:** Unlike other typical antipsychotics (like Haloperidol), Thioridazine has **low extrapyramidal side effects (EPS)** because it possesses high intrinsic anticholinergic activity [3].

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