Central Nervous System Pharmacology Practice Questions

Q: All of the following are true regarding levodopa except?

Levodopa cannot cross the blood-brain barrier.. **Explanation:** **1. Why Option C is the correct (false) statement:** The fundamental principle of treating Parkinson’s disease with Levodopa is that **Levodopa can cross the blood-brain barrier (BBB)** via the large neutral amino acid transporter (LAT). Once inside the CNS, it is decarboxylated to Dopamine [1]. Dopamine itself cannot cross the BBB; therefore, Levodopa acts as a "prodrug" to replenish central dopamine levels. **2. Analysis of other options:** * **Option A:** Levodopa has a very short plasma half-life of approximately **60 to 90 minutes**, which contributes to the need for frequent dosing and the development of motor fluctuations [1]. * **Option B:** Peripheral conversion of Levodopa to dopamine can cause **orthostatic hypotension** (via action on D1 receptors in the vasculature). **Dyskinesias** (involuntary movements) are a common long-term side effect related to fluctuating drug levels and striatal hypersensitivity [2]. * **Option D:** This describes the **"wearing-off" phenomenon** [1]. Initially, the brain can store dopamine, but as the disease progresses and nigrostriatal neurons are lost, the duration of benefit mirrors the drug's short half-life. **High-Yield Clinical Pearls for NEET-PG:** * **Carbidopa/Benserazide:** These are peripheral dopa-decarboxylase inhibitors. They do **not** cross the BBB. They are co-administered with Levodopa to prevent peripheral conversion, thereby increasing the amount of Levodopa reaching the brain and reducing peripheral side effects like nausea/vomiting [1]. * **Vitamin B6 (Pyridoxine):** It is a cofactor for decarboxylase. High doses of B6 can increase peripheral metabolism of Levodopa, reducing its efficacy (unless a decarboxylase inhibitor is used). * **Contraindication:** Levodopa can activate latent melanoma; hence, it is contraindicated in patients with suspicious undiagnosed skin lesions.

Q: Which of the following drugs is used worldwide for the maintenance therapy of opioid dependence?

Methadone. **Explanation:** The management of opioid dependence involves two phases: detoxification and **maintenance therapy**. **Methadone** is a long-acting synthetic **mu-opioid receptor agonist**. Its efficacy in maintenance therapy stems from its long half-life (24–36 hours), which prevents withdrawal symptoms and provides a "blockade effect" by inducing cross-tolerance; this prevents the euphoria associated with illicit heroin use. Because it is administered orally, it breaks the ritual of intravenous drug use, allowing for social rehabilitation. **Analysis of Options:** * **Naltrexone (Option A):** While used in opioid de-addiction, it is an **opioid antagonist**. It is used to prevent relapse in highly motivated patients who are already detoxified. It is not the primary choice for maintenance in active dependence because it can precipitate severe withdrawal if opioids are still in the system. * **Pethidine (Option B):** This is a short-acting opioid agonist. It is never used for maintenance because its metabolite, **normeperidine**, is neurotoxic and can cause seizures with chronic use. * **L-NAME (Option D):** This is a non-selective inhibitor of Nitric Oxide Synthase (NOS) used primarily in research settings to study vascular resistance; it has no role in treating opioid dependence. **High-Yield Clinical Pearls for NEET-PG:** * **Buprenorphine** (a partial mu-agonist) is another first-line agent for maintenance therapy, often preferred due to a lower risk of fatal overdose (ceiling effect). * **Clonidine** (alpha-2 agonist) is used to manage the *autonomic symptoms* of acute opioid withdrawal (tachycardia, hypertension) but is not used for maintenance. * **Naloxone** is the drug of choice for **acute opioid poisoning**.

Q: Which of the following statements about the mechanism of action of anticonvulsants is false?

Ethosuximide – K+ channel opener. ### Explanation The correct answer is **A (Ethosuximide – K+ channel opener)** because this statement is incorrect. **1. Why Option A is the correct answer (False Statement):** Ethosuximide does not act on potassium channels. Its primary mechanism of action is the **inhibition of T-type Ca²⁺ channels** in thalamic neurons. By reducing these low-threshold calcium currents, it suppresses the rhythmic 3 Hz spike-and-wave discharges characteristic of absence seizures. **2. Analysis of Incorrect Options (True Statements):** * **B. Phenytoin:** Acts by causing **use-dependent blockade of voltage-gated Na⁺ channels**. It prolongs the inactivated state of the channel, preventing high-frequency repetitive firing of action potentials. * **C. Diazepam:** As a Benzodiazepine, it acts as a **GABA-A receptor modulator**. It increases the *frequency* of chloride channel opening in the presence of GABA, leading to hyperpolarization. * **D. Gabapentin:** Although designed as a GABA structural analog, it does not act directly on GABA receptors. It binds to the **α2δ-1 subunit of voltage-gated Ca²⁺ channels**, which indirectly leads to an **increase in GABA release** and a decrease in glutamate release. **3. NEET-PG High-Yield Clinical Pearls:** * **Drug of Choice (DOC):** Ethosuximide is the DOC for **Absence Seizures** (Petit mal). However, if the patient has concomitant Generalized Tonic-Clonic Seizures (GTCS), Valproate is preferred. * **Zero-Order Kinetics:** Phenytoin follows capacity-limited (non-linear) elimination at therapeutic concentrations. * **GABA Mechanism Trick:** Remember **"Ben exerts Frequency, Barbs exert Duration"** (Benzodiazepines increase frequency; Barbiturates increase duration of Cl⁻ channel opening). * **Vigabatrin:** Irreversibly inhibits GABA-transaminase (enzyme that breaks down GABA), leading to increased GABA levels. Watch for visual field defects.

Q: A person has been consuming large quantities of alcohol daily for 20 years and is physically dependent on alcohol. Which drug should not be given to this person?

Disulfiram. **Explanation:** The core clinical issue in this scenario is the patient’s **physical dependence** on alcohol. In a chronic, heavy drinker, sudden cessation or the immediate administration of certain drugs can trigger life-threatening withdrawal or severe adverse reactions. **Why Disulfiram is the correct answer:** Disulfiram is an **aldehyde dehydrogenase inhibitor**. It works by causing the accumulation of acetaldehyde if alcohol is consumed. In a person who is physically dependent and likely has alcohol currently in their system, Disulfiram will trigger a **Disulfiram-Ethanol Reaction (DER)**. This manifests as severe flushing, tachycardia, hypotension, nausea, and potentially cardiovascular collapse. It is strictly contraindicated in patients who are currently intoxicated or have not abstained from alcohol for at least 12 hours. **Analysis of Incorrect Options:** * **Acamprosate:** An NMDA receptor antagonist used for maintaining abstinence. It does not cause a reaction with alcohol and is safe to start once withdrawal is managed. * **Naltrexone:** An opioid antagonist that reduces alcohol cravings by blocking the reward pathway. It can be started even while the patient is still drinking. * **Chlordiazepoxide:** A long-acting benzodiazepine. This is actually the **drug of choice** for managing acute alcohol withdrawal symptoms to prevent seizures and delirium tremens. **NEET-PG High-Yield Pearls:** * **Disulfiram Mechanism:** Irreversible inhibition of Aldehyde Dehydrogenase. * **Acamprosate:** Best for maintaining abstinence; excreted renally (avoid in renal failure). * **Naltrexone:** Best for reducing "heavy drinking" days; hepatotoxic (monitor LFTs). * **Wernicke’s Encephalopathy:** Always give Thiamine *before* Glucose in chronic alcoholics to prevent precipitating acute neurological decline.

Q: Which of the following statements is true for pramipexole?

All of the above. **Explanation:** Pramipexole is a **non-ergot dopamine agonist** that plays a significant role in the management of Parkinson’s disease. 1. **Mechanism of Action (Option A):** Pramipexole acts directly on postsynaptic dopamine receptors, specifically showing high affinity for the **D3 receptor subtype** (and some D2 activity). Unlike Levodopa, it does not require enzymatic conversion to be active. 2. **Clinical Utility (Option B):** It is considered a **first-line treatment** for Parkinson’s disease, particularly in younger patients (under 65 years). Initiating therapy with dopamine agonists helps delay the introduction of Levodopa, thereby postponing the onset of "on-off" fluctuations and dyskinesias. 3. **Chemical Class (Option C):** It is a **non-ergot derivative**. This is clinically significant because, unlike older ergot-derived agonists (like Bromocriptine or Pergolide), pramipexole does **not** cause retroperitoneal, pulmonary, or cardiac valvular fibrosis. **High-Yield NEET-PG Pearls:** * **Restless Leg Syndrome (RLS):** Pramipexole and Ropinirole are the drugs of choice for RLS. * **Neuroprotection:** It is thought to have antioxidant properties that may protect dopaminergic neurons. * **Side Effects:** Common side effects include nausea, hallucinations, and **impulse control disorders** (pathological gambling, hypersexuality) due to D3 stimulation in the mesolimbic pathway. * **Sudden Sleep Attacks:** Patients should be warned about the risk of falling asleep suddenly during daytime activities. * **Excretion:** Unlike Ropinirole (metabolized by CYP1A2), Pramipexole is excreted largely unchanged in the **urine**; thus, dose adjustment is required in renal failure.

Q: Which of the following mixtures of anesthetics is applied topically on the skin?

Lidocaine and prilocaine. **Explanation:** The correct answer is **Lidocaine and prilocaine**. This specific combination is known as **EMLA (Eutectic Mixture of Local Anesthetics)**. **Why Lidocaine and Prilocaine?** A "eutectic mixture" is a combination of two substances that, when mixed in a specific ratio (in this case, 1:1), results in a melting point lower than that of either individual component. While lidocaine and prilocaine are solids at room temperature, their mixture becomes a **liquid oil**. This allows for a much higher concentration of the anesthetic base to penetrate the intact keratinized skin, providing effective topical anesthesia for procedures like venipuncture, skin grafting, or minor superficial surgeries. **Analysis of Incorrect Options:** * **B, C, and D:** While these drugs are common local anesthetics, they do not form a eutectic mixture suitable for topical skin penetration when combined. * **Bupivacaine** is primarily used for infiltration, nerve blocks, and spinal anesthesia due to its long duration of action. * **Procaine** is an ester-type anesthetic with poor skin penetration and is rarely used topically. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** EMLA contains 2.5% Lidocaine and 2.5% Prilocaine. * **Application:** It must be applied under an **occlusive dressing** for at least **45–60 minutes** to achieve adequate dermal analgesia. * **Side Effects:** A classic side effect of Prilocaine (and its metabolite o-toluidine) is **Methemoglobinemia**. * **Contraindications:** EMLA should not be used on mucous membranes, broken skin, or in infants under 12 months receiving methemoglobin-inducing drugs (e.g., sulfonamides).

Q: Flumazenil antagonizes the effect of which drug?

Midazolam. **Explanation:** **Correct Answer: C. Midazolam** Flumazenil is a specific **competitive antagonist** at the benzodiazepine (BZD) binding site on the **GABA-A receptor**. Midazolam is a short-acting benzodiazepine that works by increasing the frequency of chloride channel opening. Flumazenil rapidly reverses the sedative, psychomotor, and anticonvulsant effects of benzodiazepines, making it the drug of choice for BZD overdose and reversal of conscious sedation. **Why other options are incorrect:** * **A. Ketamine:** This is a dissociative anesthetic that acts primarily as an **NMDA receptor antagonist**. It is not affected by Flumazenil. * **B. Fentanyl:** This is a potent opioid analgesic acting on **mu (μ) opioid receptors**. Its specific antagonist is **Naloxone**. * **C. Propofol:** This intravenous anesthetic acts by enhancing GABA-A currents (at a site distinct from BZDs) and blocking sodium channels. There is no specific pharmacological antagonist for Propofol; recovery depends on redistribution and metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Half-life Caution:** Flumazenil has a very short half-life (~1 hour). Since many benzodiazepines (like Diazepam) have longer half-lives, **re-sedation** can occur, requiring repeated doses or an infusion. * **Seizure Risk:** Use Flumazenil with extreme caution in patients with chronic BZD use or tricyclic antidepressant (TCA) overdose, as it can precipitate **acute withdrawal seizures**. * **Specific Antagonists Memory Aid:** * Benzodiazepines $\rightarrow$ Flumazenil * Opioids $\rightarrow$ Naloxone * Heparin $\rightarrow$ Protamine Sulfate * Iron $\rightarrow$ Desferrioxamine

Q: What is the drug of choice for an acute attack of migraine?

Sumatriptan. **Explanation:** **Sumatriptan** is the drug of choice for the management of an **acute attack of migraine** (moderate to severe). It belongs to the 'Triptan' class, which acts as selective **5-HT$_{1B/1D}$ receptor agonists**. Their therapeutic effect is mediated via three mechanisms: 1. **Vasoconstriction:** Stimulation of 5-HT$_{1B}$ receptors causes constriction of dilated cranial blood vessels. 2. **Neuropeptide Inhibition:** Stimulation of 5-HT$_{1D}$ receptors on trigeminal nerve terminals inhibits the release of pro-inflammatory neuropeptides (e.g., CGRP, Substance P). 3. **Central Inhibition:** Reduction of pain transmission in the trigeminal nucleus caudalis. **Analysis of Incorrect Options:** * **A. Methysergide:** A 5-HT antagonist used historically for **prophylaxis**, not acute attacks. It is now rarely used due to the risk of **retroperitoneal fibrosis**. * **B. Caffeine:** Often used as an adjuvant in combination with ergotamine or paracetamol to enhance absorption and provide mild vasoconstriction, but it is not the primary drug of choice. * **C. Amitriptyline:** A Tricyclic Antidepressant (TCA) which is the **drug of choice for migraine prophylaxis**, but it has no role in treating an ongoing acute attack. **High-Yield Clinical Pearls for NEET-PG:** * **Triptan Side Effects:** "Chest tightness" or "Triptan sensations" (due to coronary vasospasm). * **Contraindications:** Triptans are contraindicated in patients with Ischemic Heart Disease (IHD), Prinzmetal angina, and uncontrolled hypertension. * **Drug of Choice for Prophylaxis:** Amitriptyline (most common) or Propranolol. * **Drug of Choice for Migraine in Pregnancy:** Paracetamol (first-line); Sumatriptan is considered the safest triptan if needed.

Q: Which drug is approved for obesity treatment along with Phentermine?

Topiramate. **Explanation:** The correct answer is **Topiramate**. The FDA-approved fixed-dose combination of **Phentermine and Topiramate (extended-release)** is a potent pharmacological intervention for chronic weight management. **Mechanism of Action:** * **Phentermine:** A sympathomimetic amine that acts as an appetite suppressant by increasing norepinephrine release in the hypothalamus. * **Topiramate:** An antiepileptic drug that aids weight loss through multiple mechanisms, including GABA modulation, carbonic anhydrase inhibition, and glutamate antagonism, which collectively increase satiety and reduce cravings. **Why other options are incorrect:** * **Sibutramine:** A SNRI formerly used for obesity but **withdrawn globally** due to increased risks of non-fatal myocardial infarction and stroke (SCOUT trial). * **Fenfluramine:** An older serotonergic drug withdrawn due to risks of **cardiac valvulopathy** and pulmonary hypertension. It is currently only used in very low doses for Dravet syndrome. * **Lorcaserin:** A selective 5-HT2C receptor agonist that was **withdrawn from the market** in 2020 due to an increased risk of cancer (CAMELLIA-TIMI 61 trial). **High-Yield Clinical Pearls for NEET-PG:** * **Teratogenicity:** Topiramate is associated with **cleft lip/palate**; therefore, the combination is contraindicated in pregnancy (Category X). A negative pregnancy test is mandatory before starting therapy. * **Side Effects:** Common side effects include paresthesia, dizziness, dysgeusia (metallic taste), and insomnia. * **Other Obesity Drugs to Remember:** **Orlistat** (inhibits gastric/pancreatic lipases), **Liraglutide/Semaglutide** (GLP-1 agonists), and **Naltrexone/Bupropion** combination.

Q: A newborn developed respiratory depression in a postoperative ward. Which of the following medications can cause this?

Opioid. **Explanation:** **Correct Option: A (Opioid)** Opioids (such as Morphine or Fentanyl) are the most common pharmacological cause of postoperative respiratory depression. They act as agonists at **mu (μ) receptors** in the brainstem respiratory centers. This leads to a decreased sensitivity of the respiratory center to carbon dioxide (CO2), resulting in a reduced respiratory rate and tidal volume. Newborns are particularly vulnerable because their blood-brain barrier is immature, and their metabolic pathways for drug clearance (glucuronidation) are not fully developed, leading to prolonged drug effects. **Incorrect Options:** * **B. Propofol:** While Propofol is an intravenous anesthetic that can cause transient apnea during induction, it has a very short half-life. It is rarely the cause of sustained postoperative respiratory depression once the infusion is stopped. * **C. Furosemide:** This is a loop diuretic used to manage fluid overload. Its primary side effects are electrolyte imbalances (hypokalemia) and ototoxicity, not respiratory depression. * **D. Heparin:** This is an anticoagulant used to prevent thrombosis. Its major adverse effect is bleeding/hemorrhage; it has no direct effect on the central nervous system or respiratory drive. **Clinical Pearls for NEET-PG:** * **Antidote:** The specific antagonist for opioid-induced respiratory depression is **Naloxone** (competitive μ-receptor antagonist). * **Triad of Opioid Overdose:** Pinpoint pupils (miosis), respiratory depression, and altered mental status (coma). * **Exception:** Pethidine (Meperidine) is an opioid that typically causes **mydriasis** (due to its atropine-like action) rather than miosis. * **Safe Opioid in Renal Failure:** Fentanyl is preferred as it lacks active metabolites (unlike Morphine).

Question 1

All of the following are true regarding levodopa except?

Accepted Answer

Levodopa cannot cross the blood-brain barrier.

Answer

Half-life of levodopa is 60 to 90 minutes.

Answer

Levodopa can cause orthostatic hypotension and dyskinesias.

Answer

With continued use, the duration of benefit following an individual dose of levodopa becomes progressively shorter.

Question 2

Which of the following drugs is used worldwide for the maintenance therapy of opioid dependence?

Accepted Answer

Methadone

Answer

Naltrexone

Answer

Pethidine

Answer

L-NAME

Question 3

Which of the following statements about the mechanism of action of anticonvulsants is false?

Accepted Answer

Ethosuximide – K+ channel opener

Answer

Phenytoin – Na+ channel blocker

Answer

Diazepam – Facilitates GABA action

Answer

Gabapentin – Increases GABA release

Question 4

A person has been consuming large quantities of alcohol daily for 20 years and is physically dependent on alcohol. Which drug should not be given to this person?

Accepted Answer

Disulfiram

Answer

Acamprosate

Answer

Naltrexone

Answer

Chlordiazepoxide

Question 5

Which of the following statements is true for pramipexole?

Accepted Answer

All of the above

Answer

Acts on dopamine receptors

Answer

First-line treatment for parkinsonism

Answer

Is not an ergot alkaloid

Question 6

Which of the following mixtures of anesthetics is applied topically on the skin?

Accepted Answer

Lidocaine and prilocaine

Answer

Bupivacaine and prilocaine

Answer

Lidocaine and bupivacaine

Answer

Lidocaine and procaine

Question 7

Flumazenil antagonizes the effect of which drug?

Accepted Answer

Midazolam

Answer

Ketamine

Answer

Fentanyl

Answer

Propofol

Question 8

What is the drug of choice for an acute attack of migraine?

Accepted Answer

Sumatriptan

Answer

Methysergide

Answer

Caffeine

Answer

Amitriptyline

Question 9

Which drug is approved for obesity treatment along with Phentermine?

Accepted Answer

Topiramate

Answer

Sibutramine

Answer

Fenfluramine

Answer

Lorcaserin

Question 10

A newborn developed respiratory depression in a postoperative ward. Which of the following medications can cause this?

Accepted Answer

Opioid

Answer

Propofol

Answer

Furosemide

Answer

Heparin

Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology — MCQs

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