Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 301: Which of the following statements about Phenytoin is true?

A. It follows zero-order kinetics. (Correct Answer)
B. It is not teratogenic.
C. It is excreted unchanged in the urine.
D. It does not induce microsomal enzymes.

Explanation: ### Explanation **Correct Answer: A. It follows zero-order kinetics.** Phenytoin exhibits a unique pharmacokinetic profile known as **Michaelis-Menten kinetics** (or non-linear kinetics). At low therapeutic concentrations, it follows first-order kinetics. However, the hepatic enzymes responsible for its metabolism (CYP2C9 and CYP2C19) become easily saturated even within the therapeutic range (10–20 µg/ml). Once saturated, the rate of metabolism becomes constant regardless of the plasma concentration, shifting to **zero-order kinetics**. This is clinically significant because small dose increments can lead to disproportionately large increases in plasma levels, potentially causing toxicity. **Why the other options are incorrect:** * **B. It is not teratogenic:** Phenytoin is highly teratogenic. It causes **Fetal Hydantoin Syndrome**, characterized by craniofacial anomalies (cleft lip/palate), microcephaly, and hypoplastic phalanges/nails. * **C. It is excreted unchanged in the urine:** Phenytoin is extensively metabolized in the liver via hydroxylation and glucuronidation. Less than 5% is excreted unchanged in the urine. * **D. It does not induce microsomal enzymes:** Phenytoin is a **potent inducer** of hepatic microsomal enzymes (CYP450). It increases the metabolism of other drugs like warfarin, oral contraceptives, and theophylline, leading to decreased efficacy of these co-administered drugs. **High-Yield Clinical Pearls for NEET-PG:** * **Adverse Effects (Mnemonic: HOT MALLET):** **H**irsutism, **O**steomalacia, **T**eratogenicity, **M**egaloblastic anemia (folate deficiency), **A**taxia, **L**ymphadenopathy, **L**iver toxicity, **E**nlarged gums (Gingival Hyperplasia), and **T**oxicity (Nystagmus is the earliest sign). * **Drug of Choice:** It is a first-line drug for Generalized Tonic-Clonic Seizures (GTCS) and Focal Seizures, but it **worsens Absence Seizures**. * **Fosphenytoin:** A water-soluble prodrug of phenytoin used intravenously to avoid the local complications (Purple Glove Syndrome) associated with phenytoin injection.

Question 302: Which of the following is a centrally acting skeletal muscle relaxant?

A. Carisoprodol (Correct Answer)
B. Dantrolene
C. Gallamine
D. Succinylcholine

Explanation: **Explanation:** Skeletal muscle relaxants are broadly classified into two categories: **Peripherally acting** (acting at the neuromuscular junction or on the muscle fiber) and **Centrally acting** (acting on the spinal cord or brainstem). **1. Why Carisoprodol is Correct:** **Carisoprodol** is a centrally acting muscle relaxant. It works primarily by depressing polysynaptic reflexes in the spinal cord and subcortical areas of the brain. It is metabolized into **meprobamate**, which has sedative and anxiolytic properties. It is typically used for the relief of acute, painful musculoskeletal conditions (e.g., muscle spasms). **2. Why the Other Options are Incorrect:** * **Dantrolene:** This is a **directly acting** muscle relaxant. It acts peripherally on the muscle fiber by inhibiting the release of calcium from the sarcoplasmic reticulum (via RyR1 receptors). It is the drug of choice for Malignant Hyperthermia. * **Gallamine:** This is a **peripherally acting**, non-depolarizing (competitive) neuromuscular blocking agent. It acts by blocking nicotinic receptors ($N_m$) at the motor endplate. * **Succinylcholine:** This is a **peripherally acting**, depolarizing neuromuscular blocker. It acts as an agonist at the $N_m$ receptors, causing persistent depolarization. **High-Yield NEET-PG Pearls:** * **Classification of Centrally Acting Relaxants:** * **GABA-B Agonist:** Baclofen (Drug of choice for spasticity). * **$\alpha_2$ Agonist:** Tizanidine. * **Benzodiazepines:** Diazepam (acts via GABA-A). * **Others:** Cyclobenzaprine, Chlorzoxazone, Metaxalone. * **Meprobamate** (the metabolite of Carisoprodol) carries a high risk of abuse and physical dependence; hence, Carisoprodol is a Schedule IV controlled substance in many regions.

Question 303: Which of the following statements is FALSE about the drug treatment with Memantine?

A. It is neuroprotective
B. It has a long half-life
C. It is used in the treatment of moderate dementia
D. It is an NMDA receptor agonist (Correct Answer)

Explanation: **Explanation:** The correct answer is **D** because Memantine is an **NMDA receptor antagonist**, not an agonist [1], [2]. **1. Why Option D is False (The Mechanism):** Memantine is a low-to-moderate affinity, non-competitive **NMDA (N-methyl-D-aspartate) receptor antagonist** [1]. In Alzheimer’s disease, overstimulation of NMDA receptors by glutamate leads to excessive calcium influx, causing "excitotoxicity" and neuronal death. Memantine binds to these receptors to block this pathological activation, thereby preserving neuronal function [3]. **2. Analysis of Other Options:** * **Option A (Neuroprotective):** By preventing glutamate-induced excitotoxicity and reducing calcium-mediated cell death, Memantine exerts a neuroprotective effect on surviving neurons [3]. * **Option B (Long Half-life):** Memantine has a clinically significant long terminal half-life of approximately **60 to 80 hours**, allowing for once-daily dosing. * **Option C (Moderate Dementia):** Memantine is specifically FDA-approved for the treatment of **moderate-to-severe** Alzheimer’s disease [3]. It is often used as monotherapy or in combination with Donepezil (an acetylcholinesterase inhibitor) [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Indication:** Unlike Donepezil (used for mild-to-severe), Memantine is typically reserved for **moderate-to-severe** cases [3]. * **Side Effects:** Generally well-tolerated; the most common side effects are dizziness, headache, and confusion [3]. * **Combination Therapy:** The combination of Memantine + Donepezil (Namzaric) shows synergistic benefits in advanced dementia [1]. * **Excretion:** It is primarily excreted unchanged by the kidneys; dose adjustment is required in severe renal impairment.

Question 304: Levosulpiride acts as an antagonist of which central dopamine receptor?

A. D1
B. D2 (Correct Answer)
C. D3
D. D4

Explanation: **Explanation:** **Levosulpiride** is the levo-enantiomer of sulpiride, belonging to the **substituted benzamide** class of drugs. Its primary mechanism of action is the selective and potent **antagonism of central and peripheral Dopamine D2 receptors.** 1. **Why D2 is correct:** At low doses, levosulpiride acts as an antagonist at presynaptic D2 receptors (increasing dopamine release), while at higher doses, it blocks postsynaptic D2 receptors. In the CNS, this blockade provides antipsychotic and antidepressant effects. Peripherally, its D2 antagonism in the gastrointestinal tract makes it a potent **prokinetic agent**, used for dyspepsia and GERD. 2. **Why other options are incorrect:** * **D1:** Levosulpiride has negligible affinity for D1 receptors. D1 receptors are primarily involved in the direct pathway of the basal ganglia. * **D3 and D4:** While some atypical antipsychotics (like Clozapine) have high affinity for D4, and others (like Amisulpride) affect D3, Levosulpiride is highly selective for the **D2** subtype. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Action:** It is used both as an **atypical antipsychotic** and a **prokinetic**. * **Adverse Effects:** Due to D2 blockade in the tuberoinfundibular pathway, it can cause **hyperprolactinemia**, leading to galactorrhea, amenorrhea, and gynecomastia. * **Extrapyramidal Symptoms (EPS):** Although it is an "atypical" agent with lower risk, it can still cause Parkinsonian symptoms or tardive dyskinesia at high doses. * **Comparison:** Unlike Metoclopramide, Levosulpiride is more selective and has a better safety profile regarding CNS side effects at prokinetic doses.

Question 305: Tolerance develops to all of the following actions of opioids except:

A. Miosis (Correct Answer)
B. Analgesia
C. Euphoria
D. Nausea and vomiting

Explanation: **Explanation:** Tolerance is a pharmacological phenomenon where a subject’s reaction to a specific drug concentration is progressively reduced, requiring an increase in dosage to achieve the same effect. In the case of opioids, tolerance develops at different rates for different physiological effects. **Why Miosis is the Correct Answer:** Tolerance develops to almost all pharmacological actions of opioids **EXCEPT** for two specific effects: **Miosis** (pinpoint pupils) and **Constipation**. * **Mechanism:** Miosis occurs due to the stimulation of the Edinger-Westphal nucleus of the IIIrd cranial nerve. The lack of tolerance to miosis is clinically significant because it serves as a reliable diagnostic marker for opioid overdose, even in chronic addicts. **Analysis of Incorrect Options:** * **Analgesia (B):** Tolerance develops rapidly to the pain-relieving effects of opioids. This often necessitates dose escalation in chronic pain management or palliative care. * **Euphoria (C):** Tolerance to the "high" or euphoric effect develops very quickly, which is a primary driver for dose escalation in opioid use disorder. * **Nausea and Vomiting (D):** These effects are mediated by the stimulation of the Chemoreceptor Trigger Zone (CTZ). With repeated administration, most patients develop tolerance to these emetic effects. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for No Tolerance:** Remember **"M.C."** (like a Master of Ceremonies) — **M**iosis and **C**onstipation. * **Rate of Tolerance:** High tolerance develops to Analgesia, Euphoria, Respiratory depression, and Sedation. Moderate tolerance develops to Bradycardia. * **Lethality:** Because tolerance to respiratory depression is high but not absolute, an addict who relapses and takes their previous high dose is at extreme risk of fatal respiratory failure. * **Triad of Opioid Poisoning:** Coma, Pinpoint pupil, and Respiratory depression.

Question 306: Which of the following statements is FALSE about Naltrexone?

A. Parenterally administered (Correct Answer)
B. Used to prevent relapse of heavy drinking
C. Long acting
D. Can cause hepatotoxicity

Explanation: **Explanation:** **Naltrexone** is a long-acting opioid antagonist primarily used in the management of opioid and alcohol dependence. **1. Why Option A is False (The Correct Answer):** Naltrexone is characterized by **excellent oral bioavailability**, making it the preferred route for long-term maintenance therapy. Unlike Naloxone, which must be given parenterally due to high first-pass metabolism, Naltrexone is administered **orally** (usually 50 mg once daily). While a long-acting injectable (depot) formulation exists (Vivitrol), the statement that it is "parenterally administered" as a defining characteristic is false in the context of its standard pharmacological profile compared to Naloxone. **2. Analysis of Other Options:** * **Option B (Relapse Prevention):** Naltrexone is FDA-approved for alcohol dependence. It works by blocking the $\mu$-opioid receptors, thereby reducing the "reward" or euphoria associated with alcohol consumption and decreasing cravings. * **Option C (Long Acting):** Naltrexone has a long half-life (approx. 4 hours), but its active metabolite, **6-$\beta$-naltrexol**, has a half-life of about 13 hours. This allows for once-daily dosing, unlike Naloxone, which acts for only 30–90 minutes. * **Option D (Hepatotoxicity):** This is a known dose-related adverse effect. Liver function tests (LFTs) should be monitored in patients on long-term Naltrexone therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Naloxone vs. Naltrexone:** Remember **"O"** for **O**ral (Naltrexone) and **"O"** for **O**pioid Overdose (Naloxone - IV). * **Acamprosate:** Another drug for alcohol relapse; unlike Naltrexone, it is safe in patients with liver disease but contraindicated in renal failure. * **Pre-requisite:** Patients must be opioid-free for at least 7–10 days before starting Naltrexone to avoid precipitating acute withdrawal.

Question 307: What is the specific antagonist for benzodiazepine?

A. Flumazenil (Correct Answer)
B. Alprazolam
C. Di-isopropyl phenol
D. Naltrexone

Explanation: ### Explanation **Correct Answer: A. Flumazenil** **Mechanism of Action:** Benzodiazepines (BZDs) work by binding to a specific site on the **GABA-A receptor**, increasing the frequency of chloride channel opening, which leads to neuronal hyperpolarization. **Flumazenil** is a competitive antagonist at this specific BZD binding site. It effectively reverses the sedative effects of benzodiazepines but is less predictable in reversing BZD-induced respiratory depression. **Analysis of Incorrect Options:** * **B. Alprazolam:** This is a short-acting benzodiazepine itself, used primarily for anxiety and panic disorders. It would exacerbate, rather than antagonize, BZD toxicity. * **C. Di-isopropyl phenol:** This is the chemical name for **Propofol**, an intravenous anesthetic agent. It acts via the GABA-A receptor but at a different site than benzodiazepines and has no antagonistic properties. * **D. Naltrexone:** This is an **opioid receptor antagonist** used in the management of opioid addiction and alcohol dependence. It has no effect on the GABA-BZD receptor complex. **High-Yield Clinical Pearls for NEET-PG:** * **Half-life:** Flumazenil has a very short half-life (~1 hour). Since most BZDs (like Diazepam) last longer, **re-sedation** can occur, necessitating repeated doses or an infusion. * **Precipitated Seizures:** The most serious side effect of Flumazenil is the induction of seizures, especially in patients with chronic BZD dependence or those who have co-ingested tricyclic antidepressants (TCAs). * **Indications:** It is used for reversing BZD-induced conscious sedation and in the management of BZD overdose.

Question 308: Which of the following antiepileptic agents acts on the GABAergic system to decrease the uptake of GABA into neurons and glial cells?

A. Vigabatrin
B. Progabide
C. Gabapentin
D. Tiagabine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Tiagabine**. To understand why, we must look at the specific mechanism of GABA (Gamma-Aminobutyric Acid) modulation in the synaptic cleft. **1. Why Tiagabine is Correct:** Tiagabine is a selective inhibitor of the **GAT-1 (GABA Transporter-1)**. Under normal physiological conditions, GAT-1 is responsible for the reuptake of GABA from the synaptic cleft into neurons and glial cells (astrocytes). By blocking this transporter, Tiagabine increases the concentration and duration of GABA in the extracellular space, thereby enhancing inhibitory neurotransmission. **2. Why the Other Options are Incorrect:** * **Vigabatrin:** It acts by irreversibly inhibiting **GABA-transaminase (GABA-T)**, the enzyme responsible for the degradation of GABA. It increases GABA levels by preventing its breakdown, not by blocking uptake. * **Progabide:** It is a **GABA receptor agonist** (acting on both GABA-A and GABA-B receptors). It mimics GABA rather than affecting its transport. * **Gabapentin:** Despite its name, it does not act directly on GABA receptors or transporters. Its primary mechanism is the inhibition of **voltage-gated calcium channels** (specifically the $\alpha_2\delta$-1 subunit), which reduces the release of excitatory neurotransmitters like glutamate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tiagabine Side Effect:** It can occasionally induce non-convulsive status epilepticus in patients without epilepsy. * **Vigabatrin Warning:** Associated with **permanent bilateral visual field constriction** (requires regular perimetry). * **GABA-T vs. GAT-1:** Remember **V**igabatrin for **V**anishing (degrading) GABA via GABA-**T**, and **T**iagabine for **T**ransport (GAT-1) inhibition. * **Drug of Choice:** Tiagabine is primarily used as adjunctive therapy for focal (partial) seizures.

Question 309: Which one of the following drugs is used to treat status epilepticus?

A. Primidone
B. Carbamazepine
C. Diazepam (Correct Answer)
D. Sodium valporate

Explanation: **Explanation:** **Status Epilepticus (SE)** is a medical emergency defined as a continuous seizure lasting more than 5 minutes or recurrent seizures without recovery of consciousness between episodes. **Why Diazepam is Correct:** Benzodiazepines (BZDs) are the first-line agents for terminating acute seizures due to their rapid onset of action. **Diazepam** (or Lorazepam) works by enhancing GABA-mediated inhibitory neurotransmission through positive allosteric modulation of the $GABA_A$ receptor. This increases the frequency of chloride channel opening, leading to hyperpolarization and immediate suppression of epileptiform activity. **Why Other Options are Incorrect:** * **Primidone:** A barbiturate derivative metabolized to phenobarbital. It is used for generalized tonic-clonic and partial seizures but is not suitable for acute management due to its slow onset. * **Carbamazepine:** Primarily used for focal seizures and trigeminal neuralgia. It can actually **exacerbate** certain types of seizures (like absence or myoclonic) and is not available in an intravenous formulation for emergency use. * **Sodium Valproate:** While it is a broad-spectrum anti-epileptic used in SE as a second-line agent (after BZDs fail), it is not the initial drug of choice for immediate termination of an ongoing seizure. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC) for SE:** **Lorazepam (IV)** is often preferred over Diazepam because it has a longer duration of action in the brain (less lipid-soluble, stays in the CNS longer). * **Pre-hospital/IM route:** **Midazolam (IM)** is the preferred choice if IV access is not available. * **Sequence of Management:** 1st line: Benzodiazepines $\rightarrow$ 2nd line: Fosphenytoin/Phenytoin, Valproate, or Levetiracetam $\rightarrow$ 3rd line (Refractory): Phenobarbital, Propofol, or Midazolam infusion.

Question 310: An antiepileptic drug was found successful for treatment of tonic hind limb extension (THLE). In which type of seizure can this drug be used?

A. Absent seizures
B. General tonic clonic seizures (Correct Answer)
C. Myoclonic seizures
D. Status epilepticus

Explanation: ### Explanation The **Maximal Electroshock (MES) seizure model** is the standard laboratory method used to screen drugs for their efficacy against **Generalized Tonic-Clonic Seizures (GTCS)**. **1. Why the Correct Answer is Right:** In the MES model, an electric current is applied to an animal (usually a rodent) to induce a characteristic seizure pattern. The most critical component of this induced seizure is **Tonic Hind Limb Extension (THLE)**. If an antiepileptic drug (AED) successfully abolishes or prevents THLE in this model, it indicates that the drug is effective at preventing the spread of seizure discharge. Clinically, drugs that pass the MES test (e.g., Phenytoin, Carbamazepine) are the mainstay treatments for **Generalized Tonic-Clonic Seizures (Option B)** and focal seizures. **2. Why the Incorrect Options are Wrong:** * **Absence Seizures (Option A):** These are screened using the **Pentylenetetrazol (PTZ) seizure model**, not the MES model. Drugs effective against PTZ-induced clonic seizures (e.g., Ethosuximide, Valproate) are used for Absence seizures. * **Myoclonic Seizures (Option C):** These are also typically screened using the PTZ model or specific chemical convulsants. MES-effective drugs like Phenytoin can actually *exacerbate* myoclonic seizures. * **Status Epilepticus (Option D):** While GTCS can progress to Status Epilepticus, "Status" refers to a clinical state of prolonged seizure activity requiring emergency intravenous intervention (e.g., Lorazepam). THLE is a specific physiological marker for the *type* of seizure (GTCS), not the *duration* or *emergency status*. **3. High-Yield NEET-PG Pearls:** * **MES Model:** Tests for seizure **spread**; identifies drugs for **GTCS** (Phenytoin, Carbamazepine). * **PTZ Model:** Tests for seizure **threshold**; identifies drugs for **Absence Seizures** (Ethosuximide, Valproate). * **Kindling Model:** Used to study **Complex Partial Seizures** (Focal seizures with impaired awareness) and epileptogenesis. * **Drug of Choice (DOC):** Valproate is the DOC for most generalized seizures (GTCS, Myoclonic, Atonic), but Ethosuximide is the DOC for pure Absence seizures.

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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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