Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 201: A patient ingested an unknown substance and presented with myoclonic jerks, seizures, tachycardia, and hypotension. ECG shows a heart rate of 120/min. Arterial blood gas revealed a pH of 7.25, pCO2 of 30mm Hg, and bicarbonate ions of 15mmol/L. What is the most likely poisonous agent?

A. Amanita phalloides
B. Ethylene glycol
C. Imipramine (Correct Answer)
D. Phencyclidine

Explanation: **Explanation:** The clinical presentation of **seizures, myoclonic jerks, tachycardia, and hypotension**, combined with metabolic acidosis, is classic for **Tricyclic Antidepressant (TCA) overdose**, of which **Imipramine** is a prototype. **Why Imipramine is correct:** TCAs exert their toxic effects through four main mechanisms: 1. **Anticholinergic effects:** Tachycardia, dilated pupils, and dry skin. 2. **Inhibition of Norepinephrine reuptake:** Initial hypertension followed by hypotension due to depletion of catecholamines and $\alpha_1$-blockade. 3. **Sodium channel blockade (Quinidine-like effect):** This is the most dangerous feature, leading to QRS widening, arrhythmias, and seizures. 4. **GABA-A antagonism:** Responsible for the myoclonic jerks and seizures. The ABG shows a **High Anion Gap Metabolic Acidosis (HAGMA)**, often seen in TCA toxicity due to seizures (lactic acidosis) and cardiovascular collapse. **Why incorrect options are wrong:** * **Amanita phalloides:** Primarily causes severe gastrointestinal distress followed by fulminant hepatic failure (jaundice, coagulopathy); it does not typically present with acute seizures and tachycardia. * **Ethylene glycol:** While it causes a significant HAGMA, the hallmark is renal failure (calcium oxalate crystals in urine) and cranial nerve palsies, rather than myoclonic jerks and tachycardia. * **Phencyclidine (PCP):** Causes nystagmus (vertical/horizontal), agitation, and hypertension. While it causes seizures, it does not typically cause the profound hypotension and specific ECG changes associated with TCAs. **High-Yield Clinical Pearls for NEET-PG:** * **ECG in TCA Overdose:** Look for QRS duration >100ms and a dominant R wave in lead aVR (height >3mm). * **Antidote:** The specific treatment for TCA-induced cardiotoxicity is **Sodium Bicarbonate ($NaHCO_3$)**, which increases extracellular sodium and alkalinizes the blood to decrease the drug's affinity for sodium channels. * **Contraindication:** Avoid Physostigmine in TCA overdose as it can worsen bradycardia and precipitate asystole.

Question 202: Buspirone acts on which receptor subtype?

A. 5HT1A (Correct Answer)
B. 5HT1B
C. 5HT2
D. 5HT3

Explanation: **Explanation:** **Buspirone** is a non-benzodiazepine anxiolytic used primarily for the management of Generalized Anxiety Disorder (GAD) [1]. **1. Why Option A is Correct:** Buspirone acts as a **selective partial agonist at the 5-HT1A receptors** [2], [3]. By stimulating these presynaptic autoreceptors in the raphe nuclei, it inhibits the firing of serotonergic neurons [1]. It also acts on postsynaptic 5-HT1A receptors in the hippocampus and cortex. Unlike benzodiazepines, it does not interact with GABA receptors, which explains its lack of sedative, anticonvulsant, or muscle relaxant properties [1], [2]. **2. Why Other Options are Incorrect:** * **5-HT1B:** These receptors are primarily involved in cranial vessel vasoconstriction. Drugs like **Triptans** (e.g., Sumatriptan) act here to treat migraines [2]. * **5-HT2:** These receptors (specifically 5-HT2A) are targets for atypical antipsychotics and certain hallucinogens (like LSD). Antagonism at 5-HT2 receptors is often associated with antidepressant and antipsychotic effects [2]. * **5-HT3:** These are unique ligand-gated ion channels. Antagonists like **Ondansetron** are used as potent anti-emetics. **3. High-Yield Clinical Pearls for NEET-PG:** * **Delayed Onset:** Buspirone takes **1–2 weeks** to show therapeutic effects; it is not useful for acute anxiety or panic attacks [1], [3]. * **Safety Profile:** It has **no potential for abuse or addiction**, no withdrawal symptoms, and does not potentiate the effects of alcohol [1]. * **Side Effects:** Most common side effects include dizziness, nausea, and headache (often referred to as "nervousness") [3]. * **Metabolism:** It is metabolized by **CYP3A4**; its levels increase significantly if taken with grapefruit juice or erythromycin.

Question 203: Carbamazepine is the drug of choice in which of the following seizure types?

A. Absence seizures
B. Partial complex seizures (Correct Answer)
C. Myoclonus
D. Infantile spasms

Explanation: **Carbamazepine (CBZ)** is a first-line antiepileptic drug primarily used for focal (partial) seizures. Its mechanism of action involves blocking **voltage-gated sodium channels** in their inactivated state, which prevents high-frequency repetitive firing of action potentials [1, 2].**Why Option B is Correct:**Carbamazepine is considered the **Drug of Choice (DOC)** for **Partial (Focal) seizures**, including both simple and complex partial seizures, as well as Generalized Tonic-Clonic Seizures (GTCS). It is highly effective in stabilizing neuronal membranes against focal discharges [1, 2].**Why Other Options are Incorrect:** * **A. Absence Seizures:** Carbamazepine is contraindicated here as it can **exacerbate** absence seizures [2]. The DOC for absence seizures is Ethosuximide (or Valproate) [2]. * **C. Myoclonus:** Similar to absence seizures, Carbamazepine can worsen myoclonic jerks. The DOC for myoclonic seizures is Sodium Valproate. * **D. Infantile Spasms:** The DOC for infantile spasms (West Syndrome) is **ACTH** or Vigabatrin (especially if associated with Tuberous Sclerosis).**High-Yield Clinical Pearls for NEET-PG:** * **Trigeminal Neuralgia:** Carbamazepine is the specific **Drug of Choice** for this condition [1]. * **Auto-induction:** It induces its own metabolism (CYP3A4 induction), requiring dosage adjustments after the first few weeks of therapy. * **Side Effects:** Look for keywords like **Diplopia**, Ataxia, **SIADH** (Hyponatremia), and serious skin reactions like **Stevens-Johnson Syndrome** (associated with the HLA-B*1502 allele). * **Teratogenicity:** It can cause Neural Tube Defects (Spina Bifida).

Question 204: Triptans used in migraine headaches are agonists at which receptor?

A. 5-HT1D/1B (Correct Answer)
B. 5-HT1A
C. 5-HT3
D. 5-HT2A

Explanation: Triptans (e.g., Sumatriptan, Rizatriptan) are the first-line drugs for the treatment of acute migraine attacks [1], [2]. Their therapeutic efficacy is derived from their selective agonist action at **5-HT1B and 5-HT1D** receptors [1], [2]. 1. **5-HT1B receptors** are located on the smooth muscle of intracranial blood vessels [2]. Activation leads to **vasoconstriction** of the dilated meningeal vessels, reversing a hallmark of migraine pathophysiology [2]. 2. **5-HT1D receptors** are located presynaptically on trigeminal nerve endings [1]. Activation inhibits the release of pro-inflammatory neuropeptides (like CGRP and Substance P) [3], thereby blocking **neurogenic inflammation** [3]. **Analysis of Incorrect Options:** * **5-HT1A:** These receptors are primarily located in the CNS and are involved in mood and anxiety. **Buspirone** is a partial agonist at this site used for Generalized Anxiety Disorder. * **5-HT3:** These are ligand-gated ion channels involved in the emetic reflex. **Ondansetron** is an antagonist at this receptor used as an antiemetic. * **5-HT2A:** These receptors are involved in platelet aggregation and smooth muscle contraction. Antagonists like **Methysergide** were historically used for migraine *prophylaxis*, not acute treatment [4]. **High-Yield Clinical Pearls for NEET-PG:** * **Route of Choice:** Sumatriptan is the only triptan available for subcutaneous use (fastest onset). * **Contraindications:** Due to their vasoconstrictive properties, triptans are strictly contraindicated in patients with **Ischemic Heart Disease (Prinzmetal angina)**, uncontrolled hypertension, and peripheral vascular disease. * **Newer Class:** **Lasmiditan** is a 5-HT1F agonist used for migraine that does *not* cause vasoconstriction, making it safer for cardiac patients.

Question 205: Which one of the following drugs is effective in painful tingling sensation due to diabetic neuropathy?

A. Aspirin
B. Ibuprofen
C. Gabapentin (Correct Answer)
D. Tramadol

Explanation: **Explanation:** The correct answer is **Gabapentin**. **Why Gabapentin is correct:** Diabetic neuropathy presents as **neuropathic pain**, characterized by burning, tingling (paresthesia), or electric-shock sensations. Unlike nociceptive pain, neuropathic pain is caused by nerve damage and does not respond well to conventional analgesics [1]. Gabapentin is an **anticonvulsant** that acts as a ligand for the **$\alpha_2\delta$ subunit of voltage-gated calcium channels** in the CNS. By binding here, it decreases the influx of calcium into presynaptic neurons, thereby reducing the release of excitatory neurotransmitters (like glutamate and substance P), effectively "damping down" the overactive pain signals. **Why the other options are incorrect:** * **Aspirin & B. Ibuprofen:** These are Non-Steroidal Anti-inflammatory Drugs (NSAIDs). They work by inhibiting cyclooxygenase (COX) enzymes to reduce prostaglandin synthesis [2]. While excellent for inflammatory and somatic pain (e.g., headache, arthritis), they are largely ineffective for the structural nerve damage seen in diabetic neuropathy [1]. * **D. Tramadol:** This is a weak opioid agonist and SNRI. While it can be used as a second-line treatment for refractory neuropathic pain, it is not the first-line choice due to its side effect profile, potential for dependence, and lower efficacy compared to calcium channel ligands or SNRIs in this specific condition. **High-Yield Facts for NEET-PG:** * **First-line agents for Neuropathic Pain:** Gabapentin, Pregabalin, Amitriptyline (TCA), and Duloxetine (SNRI) [1]. * **Pregabalin** is often preferred over Gabapentin due to its linear pharmacokinetics and better bioavailability. * **Mechanism Check:** Despite its name, Gabapentin does **not** act directly on GABA receptors. * **Common Side Effects:** Dizziness, somnolence, and peripheral edema.

Question 206: Carbidopa is used in the treatment of Parkinsonism because:

A. It is a dopamine agonist
B. It decreases peripheral utilization of L-dopa (Correct Answer)
C. It increases the breakdown of L-dopa
D. It prevents the crossing of L-dopa into the brain

Explanation: **Explanation:** **Why Option B is Correct:** Levodopa (L-dopa) is a precursor to dopamine that can cross the blood-brain barrier (BBB). However, when administered alone, more than 95% of L-dopa is decarboxylated into dopamine in the peripheral tissues by the enzyme **Peripheral Dopa Decarboxylase**. Since dopamine itself cannot cross the BBB, this peripheral conversion leads to systemic side effects (nausea, vomiting, cardiac arrhythmias) and reduces the amount of drug available to reach the brain. **Carbidopa** is a peripheral dopa decarboxylase inhibitor. It does not cross the BBB; therefore, it prevents the peripheral breakdown of L-dopa, increasing its bioavailability in the CNS and allowing for a dose reduction of L-dopa by about 75-80%. **Why Other Options are Incorrect:** * **Option A:** Carbidopa has no intrinsic pharmacological activity on dopamine receptors; it is an enzyme inhibitor, not an agonist. * **Option C:** Carbidopa *inhibits* the breakdown (decarboxylation) of L-dopa; it does not increase it. * **Option D:** Carbidopa actually *facilitates* more L-dopa entering the brain by ensuring it remains intact in the systemic circulation. It does not cross the BBB itself. **High-Yield Clinical Pearls for NEET-PG:** * **The "Sinemet" Combination:** The standard ratio of Levodopa to Carbidopa is usually **4:1 or 10:1**. * **Side Effects:** While Carbidopa reduces systemic side effects (nausea/vomiting), it may actually **accentuate central side effects** like dyskinesias and hallucinations because more dopamine is being produced in the brain. * **Pyridoxine (Vitamin B6) Interaction:** Pyridoxine is a cofactor that increases peripheral decarboxylation of L-dopa. Carbidopa nullifies this interaction, making it safe to take B6 with the combination therapy.

Question 207: A client with a subarachnoid hemorrhage is prescribed a 1,000-mg loading dose of Dilantin IV. Which consideration is most important when administering this dose?

A. Therapeutic drug levels should be maintained between 20 to 30 mg/ml.
B. Rapid Dilantin administration can cause cardiac arrhythmias. (Correct Answer)
C. Dilantin should be mixed in dextrose in water before administration.
D. Dilantin should be administered through an IV catheter in the client’s hand.

Explanation: **Explanation:** **1. Why Option B is Correct:** Phenytoin (Dilantin) contains **propylene glycol** as a solvent to maintain its solubility. When administered intravenously too rapidly, propylene glycol can cause severe **cardiovascular toxicity**, leading to hypotension, bradycardia, and potentially fatal **cardiac arrhythmias** (such as heart block or ventricular fibrillation). To mitigate this risk, the rate of infusion must not exceed **50 mg/minute** in adults (or 25 mg/minute in the elderly/patients with pre-existing heart disease). Continuous ECG and blood pressure monitoring are mandatory during the loading dose. **2. Why Incorrect Options are Wrong:** * **Option A:** The therapeutic range for phenytoin is **10 to 20 mcg/mL**. Levels between 20–30 mg/mL (or mcg/mL) are considered toxic and can lead to nystagmus, ataxia, and altered mental status. * **Option C:** Phenytoin is highly alkaline and will **precipitate** immediately if mixed with Dextrose solutions. It must only be diluted in **0.9% Normal Saline** (NS). * **Option D:** Administering phenytoin into small veins (like those in the hand) is contraindicated due to the risk of **"Purple Glove Syndrome"**—a complication involving distal limb edema, discoloration, and pain that can lead to tissue necrosis. It should be given via a large-bore IV in a large vein. **NEET-PG High-Yield Pearls:** * **Mechanism:** Blocks voltage-gated sodium channels in the inactivated state (use-dependent block). * **Kinetics:** Follows **Zero-order kinetics** (capacity-limited metabolism) at therapeutic or high doses. * **Fosphenytoin:** A water-soluble prodrug of phenytoin that lacks propylene glycol; it can be given faster (150 mg/min) and has a lower risk of local tissue damage. * **Teratogenicity:** Causes **Fetal Hydantoin Syndrome** (cleft lip/palate, digital hypoplasia).

Question 208: Phase II block is seen in which of the following agents?

A. Halothane
B. Ether
C. D-tubocurarine
D. Suxamethonium (Correct Answer)

Explanation: **Suxamethonium (Succinylcholine)** is a depolarizing neuromuscular blocker (D-NMB) [2]. It produces two distinct types of neuromuscular blockade [1]:* **Phase I Block (Depolarizing):** The initial phase where the drug acts as an agonist at nicotinic receptors, causing persistent depolarization and transient fasciculations [1, 3].* **Phase II Block (Desensitizing):** Occurs with **prolonged exposure** or high doses. The membrane repolarizes, but the receptors become desensitized and unresponsive to Acetylcholine. This phase resembles a non-depolarizing block (competitive) and can be partially reversed by anticholinesterases [1, 3].**Analysis of Incorrect Options:** * **A & B (Halothane and Ether):** These are volatile inhalational anesthetics. While they can potentiate the effects of neuromuscular blockers by relaxing skeletal muscle, they do not cause a Phase II block themselves. * **C (D-tubocurarine):** This is a classic non-depolarizing (competitive) neuromuscular blocker. It produces a competitive block from the onset (characterized by "fade" on Train-of-Four monitoring) and does not transition through phases.**High-Yield Clinical Pearls for NEET-PG:** * **Pseudocholinesterase Deficiency:** Suxamethonium is metabolized by plasma pseudocholinesterase. Patients with a genetic deficiency of this enzyme experience **prolonged apnea** after a standard dose. * **Side Effects:** Hyperkalemia (dangerous in burn/trauma patients), malignant hyperthermia (treated with **Dantrolene**), and postoperative myalgia. * **Monitoring:** Phase II block is identified using a nerve stimulator; it shows a "fade" in the Train-of-Four (TOF) response, similar to non-depolarizing agents.

Question 209: Ethosuxamide is the drug of choice for which type of seizure disorder?

A. Generalized tonic-clonic seizures
B. Complex partial seizures
C. Absence seizures (Correct Answer)
D. Myoclonic seizures

Explanation: ### Explanation **Correct Answer: C. Absence seizures** **Mechanism and Rationale:** Ethosuximide is the first-line drug of choice for **Absence seizures (Petit mal)**. The underlying pathophysiology of absence seizures involves abnormal T-type calcium channel activity in thalamic neurons, which leads to the characteristic 3-Hz spike-and-wave discharges on an EEG. Ethosuximide works specifically by **inhibiting these T-type $Ca^{2+}$ channels** in the thalamus, thereby suppressing the rhythmic cortical discharges without significantly affecting other seizure types. **Analysis of Incorrect Options:** * **A. Generalized Tonic-Clonic Seizures (GTCS):** Ethosuximide is ineffective against GTCS. The drugs of choice for GTCS are Valproate, Levetiracetam, or Phenytoin/Carbamazepine. * **B. Complex Partial Seizures:** These are focal seizures with impaired awareness. The preferred treatments include Levetiracetam, Carbamazepine, or Oxcarbazepine. Ethosuximide has no role in focal epilepsy. * **C. Myoclonic Seizures:** While Valproate is the drug of choice for myoclonic seizures, Ethosuximide is narrow-spectrum and does not cover myoclonic jerks. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC) Paradox:** While Ethosuximide is the DOC for *pure* absence seizures, **Sodium Valproate** is the DOC if the patient has *concomitant* absence and GTCS (as Ethosuximide does not cover GTCS). * **Side Effects:** Remember the mnemonic **EFGH**: **E**thosuximide causes **F**atigue, **G**I distress, **H**eadache, and **H**ematological abnormalities (like Stevens-Johnson Syndrome). * **EEG Finding:** Always associate "3-Hz spike-and-wave" with Absence seizures and Ethosuximide.

Question 210: Which of the following drugs is an antagonist to diazepam?

A. Phenergan
B. Flumazenil (Correct Answer)
C. Domperidone
D. Bromocriptine

Explanation: **Explanation:** **Correct Option: B. Flumazenil** Flumazenil is a competitive **benzodiazepine (BZD) receptor antagonist**. It binds with high affinity to the specific GABA-A receptor site where benzodiazepines (like diazepam) act, effectively displacing them. It is the drug of choice for reversing benzodiazepine-induced sedation and treating acute BZD overdose. **Incorrect Options:** * **A. Phenergan (Promethazine):** An H1-receptor antagonist (antihistamine) with sedative properties. It is used for allergies, motion sickness, and nausea, but has no action on BZD receptors. * **C. Domperidone:** A peripheral D2-receptor antagonist used as an antiemetic and prokinetic. It does not cross the blood-brain barrier significantly and does not interact with GABA receptors. * **D. Bromocriptine:** A dopamine (D2) receptor agonist used in the treatment of Parkinson’s disease, hyperprolactinemia, and acromegaly. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Flumazenil antagonizes the CNS effects of benzodiazepines and "Z-drugs" (Zolpidem, Zaleplon, Eszopiclone), but it **does not** reverse the effects of barbiturates, alcohol, or opioids. * **Half-life:** Flumazenil has a very short half-life (~1 hour). Since diazepam has a much longer half-life, "re-sedation" can occur, necessitating repeated doses or an infusion. * **Contraindication:** Avoid flumazenil in patients with long-term BZD dependence or those who have co-ingested TCAs, as it can precipitate **acute withdrawal seizures**. * **Route:** Administered strictly via the intravenous (IV) route.

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