Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 501: Strychnine acts by inhibiting which of the following neurotransmitters?

A. GABA
B. Glycine (Correct Answer)
C. Acetylcholine
D. Dopamine

Explanation: **Explanation:** **Strychnine** is a potent alkaloid and a competitive antagonist of **Glycine**, which is the primary inhibitory neurotransmitter in the spinal cord. 1. **Mechanism of Action (Why B is correct):** Glycine normally binds to its receptors on motor neurons in the spinal cord to cause hyperpolarization (inhibition). Strychnine competitively blocks these glycine receptors. By inhibiting the "inhibitor," strychnine causes disinhibition of motor neurons, leading to unchecked sensory stimuli and powerful, symmetric muscle spasms (opisthotonus). 2. **Why other options are incorrect:** * **GABA (A):** While GABA is the major inhibitory neurotransmitter in the brain, its receptors are primarily targeted by drugs like benzodiazepines or toxins like **Picrotoxin** and **Bicuculline**, not strychnine. * **Acetylcholine (C):** This is the primary excitatory neurotransmitter at the neuromuscular junction. Drugs affecting it include organophosphates or botulinum toxin. * **Dopamine (D):** This is involved in the reward pathway and motor control in the basal ganglia. Its inhibition is associated with Parkinsonism or antipsychotic drug action. **High-Yield Clinical Pearls for NEET-PG:** * **Renshaw Cells:** These are inhibitory interneurons in the spinal cord that release glycine. Strychnine acts specifically at these sites. * **Clinical Presentation:** Strychnine poisoning presents with "spinal convulsions," characterized by a conscious patient experiencing generalized tonic-clonic seizures and a classic **"Risus Sardonicus"** (grimace) and **Opisthotonus** (archback), similar to Tetanus. * **Tetanus Toxin vs. Strychnine:** While both cause similar spasms, Tetanus toxin prevents the *release* of glycine, whereas Strychnine blocks the *receptor*.

Question 502: The primary site of action for addictive drugs is:

A. Nucleus raphe magnus
B. Nucleus accumbens (Correct Answer)
C. Kolliker-Fusenucleus
D. Nucleus parabrachialis

Explanation: ### Explanation **1. Why Nucleus Accumbens is Correct:** The **Nucleus Accumbens (NAc)** is the central component of the brain’s **reward circuitry** (the mesolimbic pathway) [1]. Most addictive drugs (e.g., cocaine, amphetamines, opioids, nicotine) exert their reinforcing effects by increasing the concentration of **dopamine** in the NAc [1, 2]. This dopamine surge originates from the Ventral Tegmental Area (VTA) and signals "reward," leading to the compulsive drug-seeking behavior characteristic of addiction [1]. **2. Why the Other Options are Incorrect:** * **Option A: Nucleus Raphe Magnus:** This is a major serotonergic nucleus located in the medulla. It is primarily involved in the **descending inhibition of pain** (antinociception) rather than the reward pathway. * **Option C: Kolliker-Fuse Nucleus:** Located in the pons, this nucleus is part of the **parabrachial complex** and plays a critical role in regulating the respiratory rhythm and the transition between inspiration and expiration. * **Option D: Nucleus Parabrachialis:** This area acts as a relay station for visceral sensations (like taste and pain) and autonomic control. While it has connections to the amygdala, it is not the primary site for drug addiction. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Mesolimbic Pathway:** Connects the **VTA to the Nucleus Accumbens**. This is the "pleasure center" of the brain [1, 2]. * **Dopamine (D2) Receptors:** Chronic drug abuse often leads to the downregulation of D2 receptors in the NAc, contributing to tolerance and anhedonia. * **Cocaine Mechanism:** Specifically inhibits the reuptake of dopamine in the NAc by blocking the dopamine transporter (DAT). * **Opioids:** Act by inhibiting GABAergic interneurons in the VTA, which disinhibits (activates) dopaminergic neurons projecting to the NAc.

Question 503: 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**. **Mechanism of Action:** Tiagabine is a selective inhibitor of the **GAT-1 (GABA Transporter 1)**. Under normal physiological conditions, GABA is removed from the synaptic cleft by these transporters located on neurons and glial cells (astrocytes). By blocking GAT-1, Tiagabine prevents the reuptake of GABA, thereby increasing the concentration and duration of GABA in the synaptic space, which enhances inhibitory neurotransmission. **Analysis of Incorrect Options:** * **Vigabatrin:** It acts on the GABAergic system but by a different mechanism. It is an irreversible inhibitor of **GABA transaminase (GABA-T)**, the enzyme responsible for the degradation of GABA. * **Progabide:** It is a **GABA receptor agonist** (acting on both GABA-A and GABA-B receptors) and a prodrug of GABA. It does not inhibit uptake. * **Gabapentin:** Despite its name, it does not act directly on GABA receptors or transporters. Its primary mechanism is binding to the **α2δ-1 subunit of voltage-gated calcium channels**, reducing glutamate release. **NEET-PG High-Yield Pearls:** * **Tiagabine:** Associated with the risk of "new-onset seizures" if used off-label in non-epileptic patients. * **Vigabatrin:** High-yield side effect is **permanent bilateral visual field constriction** (requires regular perimetry). It is the drug of choice for **Infantile Spasms** associated with Tuberous Sclerosis. * **GABA Transporters:** Remember that GAT-1 is the primary target in the CNS; inhibiting it specifically targets the "reuptake" phase of the neurotransmitter cycle.

Question 504: Which of the following drugs is useful in the prophylaxis of migraine?

A. Propranolol (Correct Answer)
B. Sumatriptan
C. Aspirin
D. Ergotamine

Explanation: ### Explanation The management of migraine is divided into two categories: **Abortive (Acute)** treatment and **Prophylactic (Preventive)** treatment. **Why Propranolol is Correct:** **Propranolol**, a non-selective beta-blocker, is considered the **first-line drug for the prophylaxis of migraine**. It works by stabilizing vascular tone and preventing the vasodilation of cerebral vessels. It is particularly indicated when migraine attacks occur more than 2–3 times per month or are severe enough to impair daily life. **Analysis of Incorrect Options:** * **Sumatriptan (Option B):** A 5-HT$_{1B/1D}$ receptor agonist. It is the drug of choice for **acute attacks** of migraine but is never used for prophylaxis as it has a short half-life and can cause "medication overuse headache." * **Aspirin (Option C):** An NSAID used for the **symptomatic relief** of mild to moderate acute migraine attacks. It does not prevent future occurrences. * **Ergotamine (Option D):** An ergot alkaloid used for **aborting** moderate to severe acute attacks. Due to its potent vasoconstrictive properties and side-effect profile, it is not used for long-term prevention. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylactic Drugs:** Apart from Beta-blockers (Propranolol, Timolol), other first-line agents include **Amitriptyline** (TCA), **Topiramate**, and **Valproate** (Anticonvulsants). * **DOC for Acute Attack:** Sumatriptan (Triptans). * **DOC for Migraine in Pregnancy:** Paracetamol (Acute); Magnesium or low-dose Beta-blockers (Prophylaxis, if essential). * **Contraindication:** Avoid Propranolol in patients with **Asthma** or **Heart Block**. * **Newer Agents:** **CGRP antagonists** (e.g., Erenumab) are used for prophylaxis in refractory cases.

Question 505: Which of the following is a short-acting benzodiazepine?

A. Midazolam (Correct Answer)
B. Alprazolam
C. Lorazepam
D. Diazepam

Explanation: Benzodiazepines (BZDs) are classified based on their elimination half-life into ultra-short, short, intermediate, and long-acting agents [2]. This classification is crucial for determining their clinical application, such as induction of anesthesia versus maintenance of anxiolysis. **Why Midazolam is Correct:** **Midazolam** is an ultra-short-acting benzodiazepine with an elimination half-life of approximately **1.5 to 2.5 hours**. Due to its rapid onset and short duration of action, it is the drug of choice for conscious sedation, induction of anesthesia, and as a pre-anesthetic medication [1]. Its water solubility makes it less irritating to veins compared to diazepam. **Analysis of Incorrect Options:** * **Alprazolam:** Classified as a **short-to-intermediate** acting BZD (half-life 6–12 hours). It is primarily used for panic disorders and generalized anxiety [2]. * **Lorazepam:** An **intermediate-acting** BZD (half-life 10–20 hours). It is preferred in patients with liver disease (undergoes direct conjugation) and is the drug of choice for *Status Epilepticus* [2]. * **Diazepam:** A **long-acting** BZD (half-life 20–100 hours). It has active metabolites (e.g., desmethyldiazepam) which significantly prolong its effects, making it unsuitable for rapid recovery [2]. **NEET-PG High-Yield Pearls:** 1. **Mnemonic for Short-acting BZDs (ATOMIC):** **A**lprazolam, **T**riazolam, **O**xazepam, **M**idazolam, **I**dazoxan, **C**lotiazepam [3]. 2. **Metabolism:** BZDs that bypass Phase I (oxidative) metabolism and undergo only Phase II (glucuronidation) are **L-O-T**: **L**orazepam, **O**xazepam, and **T**emazepam. These are safe in the elderly and those with liver failure. 3. **Antidote:** **Flumazenil** is a competitive BZD receptor antagonist used to reverse BZD-induced sedation or overdose.

Question 506: What are the known side effects of phenytoin?

A. Gum hypertrophy (Correct Answer)
B. Alopecia
C. Subungal exostosis
D. Onycholysis

Explanation: **Explanation:** **Phenytoin** is a widely used hydantoin derivative for the treatment of generalized tonic-clonic and focal seizures. It acts by blocking voltage-gated sodium channels in their inactive state. **1. Why Gum Hypertrophy is Correct:** **Gingival hyperplasia (Gum hypertrophy)** is one of the most common side effects of phenytoin, 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 more common in younger patients and can be minimized by maintaining strict oral hygiene. **2. Why Incorrect Options are Wrong:** * **Alopecia:** Phenytoin actually causes **hirsutism** (excessive body hair growth), not hair loss. Alopecia is more commonly associated with Sodium Valproate. * **Subungual exostosis:** This is a benign bony outgrowth under the nail bed, usually related to trauma or infection, and is not a drug-induced side effect. * **Onycholysis:** This refers to the painless separation of the nail from the nail bed. While some drugs (like tetracyclines or psoralens) cause photo-onycholysis, it is not a recognized side effect of phenytoin. **3. High-Yield Clinical Pearls for NEET-PG:** To remember Phenytoin side effects, use the mnemonic **"HOT MALAI"**: * **H** – Hirsutism, Hypertrophy of gums * **O** – Osteomalacia (due to Vitamin D metabolism interference) * **T** – Teratogenicity (**Fetal Hydantoin Syndrome**: cleft lip/palate, digital hypoplasia) * **M** – Megaloblastic anemia (due to folate deficiency) * **A** – Ataxia (sign of toxicity) * **L** – Lymphadenopathy (Pseudolymphoma) * **A** – Arrhythmias (on rapid IV injection) * **I** – Insulin inhibition (leading to hyperglycemia) **Note:** Phenytoin follows **Zero-order kinetics** (Capacity-limited elimination) at therapeutic concentrations, making its plasma levels highly sensitive to small dose adjustments.

Question 507: Which medication is considered a treatment option for juvenile myoclonic epilepsy during pregnancy?

A. Levetiracetam (Correct Answer)
B. Carbamazepine
C. Vigabatrin
D. Phenytoin

Explanation: **Explanation:** **Juvenile Myoclonic Epilepsy (JME)** is a generalized epilepsy syndrome characterized by myoclonic jerks, generalized tonic-clonic seizures (GTCS), and absence seizures. While **Valproate** is the drug of choice for JME, it is highly **teratogenic** (associated with neural tube defects and cognitive impairment) and is generally avoided in women of childbearing age. **Why Levetiracetam is correct:** Levetiracetam is a broad-spectrum antiepileptic drug (AED) effective against myoclonic seizures. It is considered a preferred alternative in pregnancy because it has a **favorable safety profile** with a significantly lower risk of major congenital malformations compared to Valproate or Phenytoin. **Why the other options are incorrect:** * **Carbamazepine:** This is a narrow-spectrum AED. It is primarily used for focal seizures and can actually **exacerbate** myoclonic and absence seizures in JME patients. * **Vigabatrin:** It is primarily used for infantile spasms (West Syndrome) and refractory focal seizures. It is associated with permanent **visual field defects** and is not a standard treatment for JME. * **Phenytoin:** Like Carbamazepine, it can worsen myoclonic jerks. Furthermore, it is associated with **Fetal Hydantoin Syndrome** (cleft lip/palate, digital hypoplasia) when used during pregnancy. **NEET-PG High-Yield Pearls:** * **Drug of Choice for JME:** Valproate (in males/non-pregnant females). * **Safest AEDs in Pregnancy:** Levetiracetam and Lamotrigine. * **Lamotrigine caveat:** While safe in pregnancy, it may also worsen myoclonic seizures in some JME patients; hence, Levetiracetam is often preferred for this specific syndrome. * **Supplementation:** All pregnant women on AEDs should receive high-dose **Folic acid (5mg)** to reduce the risk of neural tube defects.

Question 508: All of the following are true about sedatives & hypnotics except?

A. Ramelteon is a melatonin receptor agonist used to treat insomnia (Correct Answer)
B. Thiopentone is contraindicated in porphyrias
C. Midazolam is a short-acting Benzodiazepine
D. Suvorexant is an Orexin receptor antagonist used in insomnia

Explanation: **Explanation:** The question asks for the "except" statement, implying one option is technically incorrect or the others are more accurate descriptions. However, in this specific question format, **Option A** is actually a **true** statement, but it is often used in exams to test the specific mechanism of Ramelteon. If this is the designated "correct" answer in a mock setup, it usually implies a technicality or a typo in the question stem. Let’s analyze the pharmacological facts: 1. **Ramelteon (Option A):** This is a **true** statement. Ramelteon is a potent, selective agonist at **MT1 and MT2 receptors** in the suprachiasmatic nucleus. It is FDA-approved for sleep-onset insomnia and has no abuse potential (not a controlled substance). 2. **Thiopentone (Option B):** This is a **true** statement. Barbiturates like thiopentone induce **ALA synthase**, the rate-limiting enzyme in porphyrin synthesis. This can precipitate a life-threatening crisis in patients with Acute Intermittent Porphyria. 3. **Midazolam (Option C):** This is a **true** statement. Midazolam has an ultra-short elimination half-life (1.5–2.5 hours), making it ideal for conscious sedation and pre-anesthetic medication. 4. **Suvorexant (Option D):** This is a **true** statement. It is a **Dual Orexin Receptor Antagonist (DORA)** that blocks OX1R and OX2R, which are responsible for wakefulness. **Note on NEET-PG Pattern:** If all options are factually correct, the question may be flawed. However, if Option A was meant to be the "incorrect" one, examiners sometimes falsely label Ramelteon as a "GABA agonist" or "Melatonin antagonist." **High-Yield Clinical Pearls:** * **Z-drugs (Zolpidem, Zaleplon):** Act on the BZ1 (alpha-1) subunit of the GABA-A receptor; used for insomnia with less effect on sleep architecture. * **Flumazenil:** The specific antidote for Benzodiazepine overdose (competitive antagonist). * **Buspirone:** A 5-HT1A partial agonist used for Generalized Anxiety Disorder; it is **not** a sedative.

Question 509: Which of the following statements is NOT true regarding phenytoin?

A. It follows saturation kinetics.
B. It causes hyperglycemia.
C. It is teratogenic.
D. It does not block insulin release. (Correct Answer)

Explanation: ### Explanation Phenytoin is a high-yield topic for NEET-PG, known for its complex pharmacokinetics and diverse side-effect profile. **Why Option D is the Correct Answer (The "NOT True" Statement):** Phenytoin **does** block the release of insulin from the pancreas. It inhibits the secretion of insulin by interfering with calcium-mediated exocytosis in pancreatic beta cells. Because it suppresses insulin release, it can lead to elevated blood glucose levels (hyperglycemia). Therefore, the statement that it "does not block insulin release" is incorrect. **Analysis of Incorrect Options:** * **A. It follows saturation kinetics:** This is **true**. At therapeutic concentrations, the metabolic enzymes (CYP2C9/19) become saturated. The metabolism shifts from First-order to **Zero-order kinetics** (Michaelis-Menten kinetics), meaning small dose increases can lead to disproportionately large increases in plasma levels and toxicity. * **B. It causes hyperglycemia:** This is **true**. As a direct consequence of inhibiting insulin release (as explained above), patients on phenytoin may experience increased blood sugar levels. * **C. It is teratogenic:** This is **true**. Phenytoin use during pregnancy is associated with **Fetal Hydantoin Syndrome**, characterized by craniofacial anomalies (cleft lip/palate), microcephaly, and hypoplastic phalanges/nails. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Prolongs the inactivated state of voltage-gated sodium channels. * **Adverse Effects (Mnemonic: HOT MALAYALAM):** **H**irsutism, **O**steomalacia (Vitamin D metabolism interference), **T**eratogenicity, **M**egaloblastic anemia (Folate deficiency), **A**taxia, **L**ymphadenopathy, **A**rrhythmias (on rapid IV), **Y**ellow-brown skin, **A**denoma (Gingival Hyperplasia), **L**upus-like syndrome, **A**mblyopia, **M**ental retardation. * **Therapeutic Window:** 10–20 µg/ml. * **Drug Interactions:** It is a potent **enzyme inducer**, reducing the efficacy of warfarin and oral contraceptives.

Question 510: Which of the following abolishes the therapeutic effect of levodopa?

A. Thiamine
B. Carbidopa
C. Pyridoxine (Correct Answer)
D. Benserazide

Explanation: **Explanation:** The therapeutic effect of Levodopa is abolished by **Pyridoxine (Vitamin B6)** due to its role as a cofactor for the enzyme **DOPA decarboxylase**. 1. **Mechanism of Interaction:** Levodopa is a precursor to dopamine that must cross the blood-brain barrier (BBB) to be effective. However, DOPA decarboxylase exists in both the periphery and the CNS. Pyridoxine enhances the activity of peripheral DOPA decarboxylase, leading to the rapid **extracerebral conversion** of Levodopa into dopamine. Since dopamine cannot cross the BBB, this reduces the amount of Levodopa available to reach the brain, thereby abolishing its therapeutic effect and increasing peripheral side effects (like nausea and cardiac arrhythmias). 2. **Analysis of Incorrect Options:** * **Carbidopa & Benserazide:** These are peripheral DOPA decarboxylase inhibitors. They are intentionally co-administered with Levodopa (e.g., Co-careldopa) to *prevent* peripheral metabolism, thereby increasing its CNS bioavailability and reducing side effects. They enhance, rather than abolish, the effect. * **Thiamine (Vitamin B1):** This vitamin is involved in carbohydrate metabolism and Wernicke-Korsakoff syndrome; it has no significant metabolic interaction with Levodopa. **High-Yield Clinical Pearls for NEET-PG:** * **The "Pyridoxine Paradox":** While Pyridoxine interferes with Levodopa alone, it **does not** interfere with the combination of Levodopa + Carbidopa, because Carbidopa inhibits the enzyme that Pyridoxine would otherwise stimulate. * **Dietary Advice:** Patients on Levodopa monotherapy should avoid multivitamins containing high doses of B6. * **Levodopa Side Effects:** The most common peripheral side effect is nausea/vomiting (due to CTZ stimulation), while the most common central side effects are hallucinations and dyskinesias.

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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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