Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 431: What is the drug of choice for absence seizures in children younger than 3 years of age?

A. Ethosuximide (Correct Answer)
B. Carbamazepine
C. Lamotrigine
D. Phenytoin

Explanation: **Explanation:** **1. Why Ethosuximide is the Correct Answer:** Ethosuximide is the **Drug of Choice (DOC)** for absence seizures (petit mal) in children. The underlying pathophysiology of absence seizures involves abnormal T-type Calcium channels in thalamic neurons, which create rhythmic 3-Hz spike-and-wave discharges. Ethosuximide works specifically by **inhibiting these T-type $Ca^{2+}$ channels**, effectively suppressing the paroxysmal activity without the significant sedative or systemic side effects associated with broader-spectrum anticonvulsants. **2. Why the Other Options are Incorrect:** * **Carbamazepine (Option B):** This is a narrow-spectrum agent used for focal and tonic-clonic seizures. Crucially, Carbamazepine (along with Phenytoin) can **exacerbate or worsen** absence seizures and is strictly contraindicated. * **Lamotrigine (Option C):** While Lamotrigine is effective against absence seizures and is often used as a second-line agent or in mixed seizure disorders, it is not the primary DOC due to the risk of Stevens-Johnson Syndrome (SJS), especially during rapid dose titration. * **Phenytoin (Option D):** Similar to Carbamazepine, Phenytoin is a sodium channel blocker that is ineffective against absence seizures and may increase the frequency of attacks. **3. NEET-PG High-Yield Clinical Pearls:** * **DOC for Absence Seizures:** Ethosuximide (if only absence seizures are present); Valproate (if absence seizures are accompanied by Generalized Tonic-Clonic Seizures/GTCS). * **Classic EEG Finding:** 3-Hz spike-and-wave discharges. * **Side Effects of Ethosuximide:** Remember the mnemonic **EFGH** – **E**thosuximide causes **F**atigue, **G**I distress, **H**eadache (and rarely Hematologic issues/SJS). * **Valproate vs. Ethosuximide:** In children <2 years, Valproate carries a high risk of fatal hepatotoxicity, making Ethosuximide a safer profile choice for pure absence seizures.

Question 432: A drug that blocks the uptake of dopamine and norepinephrine into presynaptic nerve terminals and also blocks sodium channels in the axonal membrane is:

A. Cocaine (Correct Answer)
B. Ephedrine
C. Imipramine
D. Fluoxetine

Explanation: ### Explanation **Correct Option: A. Cocaine** Cocaine is a unique indirect-acting sympathomimetic with a dual mechanism of action. 1. **Reuptake Inhibition:** It inhibits the transporters for dopamine (DAT), norepinephrine (NET), and serotonin (SERT). By blocking the reuptake of dopamine and norepinephrine into the presynaptic terminals, it increases their concentration in the synaptic cleft, leading to CNS stimulation and peripheral sympathomimetic effects (tachycardia, hypertension). 2. **Sodium Channel Blockade:** Unlike most other psychostimulants, cocaine also acts as a **local anesthetic** by blocking voltage-gated sodium channels in the axonal membrane, preventing nerve impulse conduction. This makes it the only local anesthetic with significant vasoconstrictive properties. **Analysis of Incorrect Options:** * **B. Ephedrine:** This is a mixed-acting sympathomimetic. It works by directly stimulating alpha and beta receptors and by displacing stored norepinephrine from vesicles (non-exocytotic release). It does not block sodium channels. * **C. Imipramine:** A Tricyclic Antidepressant (TCA) that blocks the reuptake of norepinephrine and serotonin. While TCAs can affect cardiac sodium channels in toxic doses (leading to QRS prolongation), they are not classified as drugs that block axonal sodium channels for anesthetic purposes in this context. * **D. Fluoxetine:** A Selective Serotonin Reuptake Inhibitor (SSRI). It selectively blocks the reuptake of serotonin (SERT) and has negligible effects on dopamine/norepinephrine reuptake or sodium channels. **High-Yield Clinical Pearls for NEET-PG:** * **Cocaine Toxicity:** Presents with "Sympathomimetic Toxidrome" (mydriasis, tremors, tachycardia, hypertension, and psychosis). * **Drug of Choice for Cocaine-induced HTN:** Benzodiazepines (first-line) or Phentolamine. **Avoid pure Beta-blockers** (like Propranolol) as they cause "unopposed alpha-stimulation," leading to a hypertensive crisis. * **Formulation:** Cocaine is the only naturally occurring local anesthetic (derived from *Erythroxylum coca*).

Question 433: Which of the following is a contraindication for the use of triptans?

A. Ischemic heart disease
B. Epilepsy
C. Hepatic failure
D. All of the above (Correct Answer)

Explanation: Triptans (e.g., Sumatriptan) are **5-HT$_{1B/1D}$ receptor agonists** used as first-line agents for the acute management of migraine. Their mechanism involves vasoconstriction of cranial blood vessels and inhibition of pro-inflammatory neuropeptide release. 1. **Ischemic Heart Disease (IHD):** This is the most critical contraindication. Because triptans cause vasoconstriction via 5-HT$_{1B}$ receptors, they are not selective to cranial vessels and can cause **coronary vasospasm** [1]. This can precipitate myocardial infarction or angina in patients with pre-existing IHD, Prinzmetal angina, or uncontrolled hypertension [1]. 2. **Epilepsy:** Triptans can lower the seizure threshold. While not as common as cardiovascular risks, they are generally avoided or used with extreme caution in patients with a history of seizures. 3. **Hepatic Failure:** Most triptans (except Almotriptan) undergo significant hepatic metabolism (primarily via MAO-A or Cytochrome P450). In hepatic failure, drug levels can rise to toxic levels, increasing the risk of systemic vasoconstriction and Serotonin Syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Selective agonists at 5-HT$_{1B}$ (vascular) and 5-HT$_{1D}$ (presynaptic trigeminal nerve endings) receptors. * **Drug of Choice:** Sumatriptan is the DOC for acute migraine and cluster headaches. * **Serotonin Syndrome:** Risk increases when triptans are co-administered with SSRIs, SNRIs, or MAO inhibitors. * **Contraindications:** Also include Peripheral Vascular Disease (PVD), history of Stroke/TIA, and Wolff-Parkinson-White syndrome (specifically for Zolmitriptan).

Question 434: What is the drug of choice for controlling severe pain in cancer patients?

A. Morphine (Correct Answer)
B. Diclofenac
C. Ibuprofen
D. Codeine

Explanation: **Explanation:** **Morphine** is considered the "Gold Standard" and the drug of choice for managing severe, chronic pain in cancer patients [3]. This is based on the **WHO Analgesic Ladder**, which recommends a step-wise approach to pain management [2], [5]. Morphine is a strong opioid agonist that primarily acts on **mu (μ) receptors** in the CNS to provide potent analgesia [4]. It has no "ceiling effect," meaning the dose can be titrated upwards to achieve pain relief as the disease progresses, provided side effects are managed [1]. **Analysis of Incorrect Options:** * **Diclofenac & Ibuprofen (NSAIDs):** These are Step 1 drugs on the WHO ladder. They are effective for mild-to-moderate pain or bone metastasis (due to prostaglandin inhibition) but are insufficient for the severe, deep-seated visceral pain typical of advanced cancer. * **Codeine:** This is a weak opioid (Step 2). It is used for moderate pain but has a "ceiling effect" and is often associated with significant constipation without providing the high-potency relief required for severe cancer pain. **High-Yield Clinical Pearls for NEET-PG:** * **Route of Choice:** For chronic cancer pain, the **oral route** is preferred (sustained-release formulations) [5]. * **Side Effects:** While tolerance develops to most side effects, it **never** develops to **miosis (pin-point pupils)** and **constipation** [4]. * **Mydriatic Exception:** Pethidine (Meperidine) is an opioid that causes mydriasis (due to its atropine-like action), unlike Morphine. * **Antidote:** Naloxone is the specific antagonist used for opioid overdose.

Question 435: Which one of the following is an indication for valproate?

A. Prophylaxis for migraine
B. Complex partial seizures
C. Acute mania
D. All of the above (Correct Answer)

Explanation: **Explanation:** Valproate (Sodium Valproate/Valproic Acid) is a broad-spectrum anti-epileptic drug with a multifaceted mechanism of action. It increases GABA levels (by inhibiting GABA transaminase), blocks voltage-gated sodium channels, and inhibits T-type calcium channels. This versatility makes it effective across various neurological and psychiatric conditions. * **Option A (Prophylaxis for migraine):** Valproate is a first-line agent for the prophylactic treatment of migraine. It reduces the frequency and severity of attacks, likely by modulating GABAergic neurotransmission and reducing neuronal hyperexcitability. * **Option B (Complex partial seizures):** As a broad-spectrum anticonvulsant, Valproate is effective against almost all seizure types, including generalized (Tonic-clonic, Absence, Myoclonic) and focal seizures (Simple and Complex partial). * **Option C (Acute mania):** Valproate acts as a potent mood stabilizer. It is FDA-approved for the treatment of acute manic or mixed episodes associated with Bipolar Disorder, often showing efficacy in patients who do not respond to Lithium. Since all three clinical scenarios are established indications, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Valproate is the DOC for **Myoclonic seizures** and is preferred for idiopathic generalized epilepsies. * **Teratogenicity:** It is highly teratogenic, causing **Neural Tube Defects** (e.g., Spina Bifida). Avoid in pregnancy; if essential, supplement with high-dose Folic Acid. * **Side Effects:** Remember the mnemonic **VALPROATE**: **V**omiting, **A**lopecia (curly hair), **L**iver toxicity (Hepatotoxicity), **P**ancreatitis, **R**etention of fat (Weight gain), **O**edema, **A**taxia, **T**hrombocytopenia, and **E**ncephalopathy (due to Hyperammonemia).

Question 436: Which of the following statements regarding Phenytoin is FALSE?

A. Induces microsomal enzymes
B. At very low doses, zero-order kinetics occurs (Correct Answer)
C. Higher the dose, higher is the half-life
D. Highly protein-bound

Explanation: ### Explanation **Phenytoin** is a classic example of a drug that exhibits **Saturation (Capacity-limited) Kinetics**, also known as Michaelis-Menten kinetics. **Why Option B is False (The Correct Answer):** Phenytoin follows **First-order kinetics** (constant fraction of drug eliminated per unit time) at **low therapeutic doses** because the metabolizing liver enzymes (CYP2C9/19) are not yet saturated. As the plasma concentration increases and approaches the therapeutic range, the enzymes become saturated. At this point, the drug shifts to **Zero-order kinetics** (constant amount of drug eliminated per unit time). Therefore, the statement that zero-order occurs at "very low doses" is pharmacologically incorrect. **Analysis of Other Options:** * **Option A (True):** Phenytoin is a potent **inducer of hepatic microsomal enzymes** (CYP450). This leads to significant drug interactions, such as decreasing the efficacy of oral contraceptives and warfarin. * **Option C (True):** Because of saturation kinetics, once the elimination pathways are overwhelmed, the rate of metabolism cannot increase with the dose. Consequently, the **plasma half-life increases** as the dose/concentration increases. * **Option D (True):** Phenytoin is **highly protein-bound (~90%)**, primarily to albumin. Conditions like uremia or hypoalbuminemia can increase the free (active) fraction of the drug, leading to toxicity even at "normal" total plasma levels. **High-Yield Clinical Pearls for NEET-PG:** * **Therapeutic Window:** 10–20 µg/ml. Small dose increments can lead to disproportionate increases in plasma levels (due to the shift to zero-order). * **Teratogenicity:** Causes **Fetal Hydantoin Syndrome** (cleft lip/palate, digital hypoplasia). * **Side Effects Mnemonic (PHENYTOIN):** **P**-450 induction, **H**irsutism, **E**nlarged gums (Gingival Hyperplasia), **N**ystagmus (earliest sign of toxicity), **Y**ellow-brown skin, **T**eratogenicity, **O**steomalacia, **I**nterference with B12/Folate (Megaloblastic anemia), **N**europathy.

Question 437: Which of the following is NOT a monoamine oxidase inhibitor?

A. Tranylcypromine
B. Isocarboxazide
C. Phenelzine
D. Maprotiline (Correct Answer)

Explanation: **Explanation:** The correct answer is **Maprotiline** because it belongs to the class of **Tetracyclic Antidepressants (TeCAs)**, not Monoamine Oxidase Inhibitors (MAOIs). Its primary mechanism of action is the selective inhibition of norepinephrine reuptake; it has minimal effect on serotonin or dopamine reuptake and does not inhibit the MAO enzyme. **Analysis of Options:** * **Tranylcypromine:** A non-selective, irreversible MAO inhibitor. It is a derivative of cyclopropylamine and is structurally related to amphetamine. * **Isocarboxazide:** A non-selective, irreversible MAO inhibitor belonging to the hydrazine group. * **Phenelzine:** Another hydrazine-derivative, non-selective, irreversible MAO inhibitor used primarily in treatment-resistant depression. **Clinical Pearls for NEET-PG:** 1. **Classification of MAOIs:** * **Non-selective (MAO-A & B):** Phenelzine, Isocarboxazide, Tranylcypromine. * **Selective MAO-A:** Moclobemide (Reversible Inhibitor of MAO-A or RIMA). * **Selective MAO-B:** Selegiline, Rasagiline (used in Parkinson’s disease). 2. **Cheese Reaction:** MAOIs (especially non-selective ones) can cause a hypertensive crisis when taken with tyramine-rich foods (cheese, wine) due to the displacement of norepinephrine. 3. **Drug Interactions:** Always maintain a "washout period" (usually 2 weeks) when switching between MAOIs and SSRIs to prevent **Serotonin Syndrome**. 4. **Maprotiline Specifics:** It is known for a higher incidence of **seizures** compared to other antidepressants, especially at high doses.

Question 438: Which of the following statements is true regarding carbamazepine?

A. Used in the treatment of trigeminal neuralgia. (Correct Answer)
B. Carbamazepine is a potent enzyme inducer.
C. Can cause megaloblastic anemia.
D. It is the drug of choice for status epilepticus.

Explanation: **Explanation:** **Carbamazepine** is a sodium channel blocker that stabilizes hyperexcitable nerve membranes. 1. **Why Option A is Correct:** Carbamazepine is the **drug of choice (DOC) for Trigeminal Neuralgia**. It works by reducing the frequency of paroxysmal discharges in the trigeminal nerve, providing significant pain relief for this specific neuropathic condition. 2. **Analysis of Incorrect Options:** * **Option B:** While Carbamazepine is indeed a potent **enzyme inducer** (inducing CYP3A4 and its own metabolism—autoinduction), the question asks for the "true statement" in a context where Option A is the most clinically definitive primary indication. *Note: In many competitive exams, if multiple statements are technically true, the one defining the drug's primary clinical utility is prioritized.* * **Option C:** Carbamazepine is associated with **aplastic anemia** and agranulocytosis, not megaloblastic anemia. Megaloblastic anemia is a classic side effect of **Phenytoin** (due to folate interference). * **Option D:** The drug of choice for **Status Epilepticus** is intravenous **Lorazepam** (or Diazepam), followed by Fosphenytoin/Phenytoin. Carbamazepine is contraindicated in absence and myoclonic seizures as it may worsen them. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Blocks voltage-gated $Na^+$ channels in the inactivated state. * **DOC for:** Trigeminal neuralgia and Partial (Focal) seizures. * **Side Effects:** Diplopia, Ataxia, **SIADH** (Hyponatremia), and potentially fatal skin reactions like **Stevens-Johnson Syndrome** (associated with the HLA-B*1502 allele). * **Teratogenicity:** Can cause neural tube defects (Spina Bifida).

Question 439: D2 receptors are maximally seen in which area?

A. Frontal lobe
B. Nucleus accumbens
C. Cerebellum
D. Corpus striatum (Correct Answer)

Explanation: **Explanation:** The correct answer is **Corpus striatum**. Dopamine D2 receptors are G-protein coupled receptors (Gi/o) that inhibit adenylyl cyclase. In the brain, they are most densely concentrated in the **Corpus striatum** (comprising the caudate nucleus and putamen). This area is the primary hub of the **Nigrostriatal pathway**, where dopamine is released from the substantia nigra pars compacta to modulate motor control. **Analysis of Options:** * **Corpus striatum (Correct):** It contains the highest density of D2 receptors. This is clinically significant because the blockade of these receptors by antipsychotics (especially first-generation) leads to Extrapyramidal Side Effects (EPS). * **Nucleus accumbens:** While this area is rich in D2 receptors, it is part of the **Mesolimbic pathway** (involved in reward and addiction). The density here is high but remains secondary to the corpus striatum. * **Frontal lobe:** This area is primarily associated with the **Mesocortical pathway**. It contains a higher ratio of D1 and D4 receptors compared to D2. * **Cerebellum:** This region has a very low density of dopamine receptors, as its primary neurotransmitters are GABA and Glutamate. **Clinical Pearls for NEET-PG:** 1. **D2 Blockade:** Antipsychotic potency is directly correlated with their binding affinity for D2 receptors. 2. **Nigrostriatal Pathway:** Blockade here causes EPS (Parkinsonism, Akathisia, Dystonia). 3. **Mesolimbic Pathway:** Blockade here treats the "positive symptoms" of Schizophrenia. 4. **Tuberoinfundibular Pathway:** D2 blockade here leads to hyperprolactinemia (Dopamine is the Prolactin Inhibiting Factor).

Question 440: Levodopa is contraindicated in which of the following conditions?

A. Hypertension
B. Multiple sclerosis
C. Pheochromocytoma
D. Angle closure glaucoma (Correct Answer)

Explanation: **Explanation:** **Why Angle Closure Glaucoma is the Correct Answer:** Levodopa is a precursor of dopamine, which is further metabolized into norepinephrine and epinephrine. These catecholamines act on **$\alpha_1$-adrenergic receptors**, causing mydriasis (dilation of the pupil). In patients with a narrow iridocorneal angle, mydriasis can further obstruct the outflow of aqueous humor, leading to a dangerous and acute rise in intraocular pressure (IOP). Therefore, it is strictly contraindicated in **angle-closure (narrow-angle) glaucoma**. However, it can be used cautiously in chronic open-angle glaucoma if IOP is well-controlled. **Analysis of Incorrect Options:** * **A. Hypertension:** While Levodopa can cause transient cardiac arrhythmias or orthostatic hypotension, it is not an absolute contraindication. Blood pressure should be monitored, but it is not a primary contraindication like narrow-angle glaucoma. * **B. Multiple Sclerosis:** This is a demyelinating disease of the CNS. Levodopa has no significant interaction with the pathophysiology of MS and is neither indicated nor contraindicated. * **C. Pheochromocytoma:** While catecholamine-producing tumors require caution with drugs affecting dopamine/norepinephrine, the most definitive and high-yield contraindication among the choices provided for NEET-PG is angle-closure glaucoma. **Clinical Pearls for NEET-PG:** * **Absolute Contraindications of Levodopa:** Angle-closure glaucoma, undiagnosed skin lesions (may activate **malignant melanoma**), and use of **Non-selective MAO inhibitors** (risk of hypertensive crisis). * **Vitamin Interaction:** Pyridoxine (Vitamin B6) increases the peripheral decarboxylation of Levodopa, reducing its CNS availability (unless given with Carbidopa). * **Side Effects:** The "On-Off" phenomenon and dyskinesias are common long-term complications.

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