Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 131: Which of the following is not a side effect of paroxetine?

A. Premature ejaculation (Correct Answer)
B. Erectile dysfunction
C. Decreased libido
D. Diarrhea

Explanation: ### Explanation **Paroxetine** is a potent Selective Serotonin Reuptake Inhibitor (SSRI). Understanding its side effect profile is crucial for NEET-PG, as SSRIs are first-line treatments for various psychiatric disorders but carry distinct adverse effects. **Why "Premature Ejaculation" is the Correct Answer:** Paroxetine does **not** cause premature ejaculation; in fact, it causes the exact opposite: **delayed ejaculation** [2]. By increasing serotonin levels in the synaptic cleft, SSRIs inhibit the spinal ejaculatory reflex. Because of this specific "side effect," paroxetine is actually used off-label as a primary pharmacological **treatment** for premature ejaculation. **Analysis of Incorrect Options:** * **B & C (Erectile Dysfunction & Decreased Libido):** Sexual dysfunction is the most common long-term side effect of SSRIs (occurring in up to 60% of patients). This includes decreased libido, erectile dysfunction, and anorgasmia in both men and women [1], [2]. * **D (Diarrhea):** Serotonin receptors (5-HT3 and 5-HT4) are abundant in the gastrointestinal tract. Increasing serotonin levels leads to GI upset, most commonly manifesting as nausea, vomiting, and diarrhea, especially during the initiation of therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Most Sedating SSRI:** Paroxetine (due to mild anticholinergic activity). * **Shortest Half-life SSRI:** Paroxetine (highest risk of **Discontinuation Syndrome**) [3]. * **Teratogenicity:** Paroxetine is generally avoided in pregnancy as it is associated with **fetal cardiac defects** (Category D) [3]. * **Weight Gain:** Among SSRIs, paroxetine is most frequently associated with significant weight gain [3].

Question 132: MPTP (2'-methyl-1,2,3,4'-tetrahydro-carbolines) is a causative factor for which of the following conditions?

A. Parkinson's disease (Correct Answer)
B. Schizophrenia
C. Alzheimer's disease
D. Huntington's chorea

Explanation: **Explanation:** **MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)** is a potent neurotoxin that specifically targets the dopaminergic neurons in the **substantia nigra pars compacta**, leading to a clinical syndrome indistinguishable from **Parkinson’s disease**. **Mechanism of Action:** MPTP is lipophilic and crosses the blood-brain barrier. In the brain, it is converted by the enzyme **MAO-B** (in glial cells) into the toxic metabolite **MPP+** (1-methyl-4-phenylpyridinium). MPP+ is then taken up by dopaminergic neurons via the dopamine transporter (DAT), where it inhibits **Complex I of the mitochondrial electron transport chain**. This leads to ATP depletion, oxidative stress, and subsequent neuronal death. **Why other options are incorrect:** * **Schizophrenia:** Primarily associated with an overactivity of dopamine in the mesolimbic pathway, not the destruction of nigrostriatal neurons. * **Alzheimer’s disease:** Characterized by the accumulation of amyloid-beta plaques and tau tangles, primarily affecting cholinergic neurons in the Nucleus Basalis of Meynert. * **Huntington’s chorea:** An autosomal dominant disorder caused by CAG repeats, leading to the atrophy of the caudate nucleus and GABAergic neuron loss. **High-Yield Clinical Pearls for NEET-PG:** * **Discovery:** MPTP was discovered accidentally in the 1980s among illicit drug users ("frozen addicts") who injected a synthetic heroin analog (MPPP) contaminated with MPTP. * **Protective Agent:** **Selegiline** (a MAO-B inhibitor) can prevent the conversion of MPTP to MPP+, thereby protecting against MPTP-induced parkinsonism. * **Research Use:** MPTP is widely used in laboratories to create animal models of Parkinson’s disease to study new treatments.

Question 133: All of the following are true about Triptans except:

A. They are the drug of choice in the acute attack of migraine.
B. Frovatriptan is the longest acting triptan.
C. Sumatriptan can also be given subcutaneously.
D. They act by inhibiting 5HT 1B / 1D receptors. (Correct Answer)

Explanation: The core mechanism of Triptans involves **agonism**, not inhibition. Triptans are selective **5-HT$_{1B/1D}$ receptor agonists** [1, 2]. Their therapeutic effect in migraine is mediated through three mechanisms:1. **Vasoconstriction:** Activation of 5-HT$_{1B}$ receptors on intracranial blood vessels reverses vasodilation [1].2. **Neuropeptide Inhibition:** Activation of 5-HT$_{1D}$ receptors on trigeminal nerve endings inhibits the release of pro-inflammatory peptides (like CGRP and Substance P) [2].3. **Central Inhibition:** They inhibit pain transmission in the trigeminal nucleus caudalis.#### Analysis of Options:* **Option D (Correct):** This is the false statement. Triptans **stimulate** (act as agonists) rather than inhibit these receptors.* **Option A:** Triptans are indeed the **Drug of Choice (DOC)** for moderate-to-severe acute migraine attacks when NSAIDs fail [1].* **Option B:** **Frovatriptan** has the longest half-life (~26 hours), making it useful for preventing menstrual migraine [3].* **Option C:** **Sumatriptan** is the prototype and is available via oral, nasal, and **subcutaneous** routes [1, 3]. The SC route offers the fastest onset of action (approx. 10–15 mins) [3].#### High-Yield NEET-PG Pearls:* **Fastest acting triptan:** Rizatriptan (oral) or Sumatriptan (SC) [3].* **Contraindications:** Since they cause vasoconstriction, they are strictly contraindicated in patients with **Ischemic Heart Disease (IHD)**, Prinzmetal angina, and uncontrolled hypertension [3].* **Sumatriptan** has the lowest oral bioavailability among the group.* **Almotriptan** has the best oral bioavailability.

Question 134: Which of the following actions is ascribed to delta type of opioid receptors?

A. Supraspinal analgesia (Correct Answer)
B. Respiratory depression
C. Euphoria
D. Reduced intestinal motility

Explanation: **Explanation:** Opioid receptors are G-protein coupled receptors (GPCRs) categorized into three main types: **Mu (μ), Kappa (κ), and Delta (δ)**. Understanding their distinct physiological effects is a high-yield topic for NEET-PG. **1. Why Supraspinal Analgesia is Correct:** Delta (δ) receptors are primarily located in the cerebral cortex and the dorsal horn of the spinal cord. Their activation produces both **spinal and supraspinal analgesia**. While Mu receptors are the primary mediators of analgesia, Delta receptors contribute significantly to the modulation of pain perception at the supraspinal level. **2. Analysis of Incorrect Options:** * **Respiratory Depression (Option B):** This is the most serious side effect of opioid use and is primarily mediated by **Mu (μ₂)** receptors. Delta receptors have a negligible role in respiratory depression. * **Euphoria (Option C):** This "rewarding" effect is mediated by **Mu (μ)** receptors. In contrast, Kappa (κ) receptor stimulation often leads to the opposite effect—dysphoria. * **Reduced Intestinal Motility (Option D):** Constipation is a classic side effect mediated predominantly by **Mu (μ)** receptors located in the myenteric plexus of the gastrointestinal tract. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mu (μ):** Responsible for most clinical effects: Analgesia (spinal/supraspinal), respiratory depression, euphoria, sedation, miosis, and physical dependence. * **Kappa (κ):** Responsible for spinal analgesia, **dysphoria**, psychotomimetic effects (hallucinations), and **miosis**. * **Delta (δ):** Responsible for spinal/supraspinal analgesia and potentially affective (emotional) behavior. * **Pure Antagonist:** **Naloxone** and **Naltrexone** antagonize all three receptor types, though they have the highest affinity for Mu receptors.

Question 135: Baclofen is used in which of the following conditions?

A. Parkinsonism
B. Epilepsy
C. Spasticity (Correct Answer)
D. Stroke

Explanation: **Explanation:** **Baclofen** is a centrally acting skeletal muscle relaxant that acts as a selective **GABA-B receptor agonist**. It works by increasing potassium conductance, leading to hyperpolarization of neurons. This inhibits both monosynaptic and polysynaptic reflexes at the spinal cord level, effectively reducing muscle tone and frequency of spasms. * **Why Spasticity is Correct:** Baclofen is the drug of choice for managing **spasticity** associated with spinal cord injuries, multiple sclerosis, and amyotrophic lateral sclerosis (ALS). It is preferred because it causes less sedation compared to benzodiazepines. **Analysis of Incorrect Options:** * **Parkinsonism:** This is a movement disorder caused by dopamine deficiency in the nigrostriatal pathway. Treatment involves dopaminergic agents (Levodopa) or anticholinergics, not GABA agonists. * **Epilepsy:** Baclofen is generally **avoided** in patients with epilepsy as it can lower the seizure threshold and potentially worsen the condition. * **Stroke:** While stroke can lead to spasticity (upper motor neuron lesion), Baclofen is less effective for spasticity of cerebral origin compared to spinal origin. Furthermore, it is not a primary treatment for the stroke itself. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** GABA-B agonist (G-protein coupled receptor). * **Administration:** Can be given orally or via an **intrathecal pump** for severe refractory spasticity. * **Withdrawal Warning:** Abrupt discontinuation of Baclofen can lead to life-threatening withdrawal symptoms, including hallucinations, seizures, and rebound spasticity. * **Comparison:** Unlike Dantrolene (which acts directly on the muscle/Ryanodine receptors), Baclofen acts on the **spinal cord**.

Question 136: All of the following statements about phenytoin are true except:

A. It follows saturation kinetics
B. Anti-seizure activity closely resembles plasma concentration
C. Does not depress CNS
D. Cerebellar degeneration occurs on long-term administration (Correct Answer)

Explanation: ### Explanation **Phenytoin** is a high-yield topic in NEET-PG, known for its complex pharmacokinetics and specific side effect profile. **1. Why Option D is the Correct Answer (The "Except" Statement):** While **acute phenytoin toxicity** characteristically presents with cerebellar signs such as **nystagmus, ataxia, and dysarthria**, these are typically reversible upon dose reduction. The statement that "cerebellar degeneration occurs on long-term administration" is considered clinically inaccurate in the context of standard therapeutic use. While rare cases of permanent atrophy have been reported in extreme, chronic toxicity, it is not a standard feature of long-term administration compared to other well-known side effects like gingival hyperplasia or hirsutism. **2. Analysis of Incorrect Options:** * **Option A (True):** Phenytoin follows **Zero-order (Saturation) kinetics** at therapeutic or high doses. Small dose increments can lead to disproportionately large increases in plasma levels because the metabolizing enzymes (CYP2C9/19) become saturated. * **Option B (True):** Phenytoin has a narrow therapeutic index (10–20 µg/ml). Its anti-seizure efficacy and toxicity correlate closely with its free plasma concentration. * **Option C (True):** Unlike phenobarbital or benzodiazepines, phenytoin is relatively **non-sedating**. It stabilizes neuronal membranes by blocking voltage-gated sodium channels without causing generalized CNS depression. **3. Clinical Pearls for NEET-PG:** * **Teratogenicity:** Causes **Fetal Hydantoin Syndrome** (hypoplastic phalanges, cleft lip/palate). * **Side Effects Mnemonic (HOT MALAI):** **H**irsutism, **O**steomalacia, **T**eratogenicity, **M**egaloblastic anemia, **A**taxia, **L**ymphadenopathy, **A**rrhythmias (on rapid IV), **I**nsulin inhibition (hyperglycemia). * **Gingival Hyperplasia:** Due to increased expression of Platelet-Derived Growth Factor (PDGF). * **Drug Interactions:** It is a potent **enzyme inducer**, decreasing the efficacy of oral contraceptives and warfarin.

Question 137: Lorazepam is preferred over Diazepam in the treatment of status epilepticus because:

A. It has a longer half-life than diazepam.
B. It has a more sustained action due to lower lipid solubility and slower distribution. (Correct Answer)
C. It has faster clearance.
D. It has more bioavailability.

Explanation: **Explanation:** In the management of status epilepticus, the choice between Lorazepam and Diazepam is dictated by their **pharmacokinetic profiles**, specifically their lipid solubility and redistribution patterns. **Why Option B is Correct:** While Diazepam is highly lipid-soluble and enters the brain rapidly, it also **redistributes** out of the brain into peripheral fat stores very quickly (within 15–30 minutes). This leads to a rapid decline in its anticonvulsant effect, necessitating repeated dosing. **Lorazepam**, however, is less lipid-soluble. It enters the brain slightly slower than diazepam but remains in the CNS for a significantly longer duration because it does not redistribute to peripheral tissues as rapidly. Therefore, Lorazepam provides a **more sustained anticonvulsant action** (up to 12–24 hours), making it the preferred first-line agent. **Analysis of Incorrect Options:** * **Option A:** Diazepam actually has a much longer elimination half-life (30–60 hours) than Lorazepam (10–20 hours). However, half-life does not determine the duration of action in acute seizure management; redistribution does. * **Option C:** Faster clearance would be a disadvantage in status epilepticus as it would shorten the duration of the drug's effect. * **Option D:** Both drugs have excellent bioavailability, but this is irrelevant in status epilepticus where the **intravenous (IV)** route is standard. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Lorazepam (IV) is the DOC for acute status epilepticus. * **Home/Pre-hospital management:** Midazolam (Intranasal/Buccal) or Diazepam (Rectal) are preferred. * **Metabolism:** Lorazepam is metabolized via glucuronidation (Phase II), making it safer in patients with liver dysfunction compared to Diazepam (Phase I metabolism).

Question 138: Which of the following neuromuscular blocking agents has the least effect on the cardiovascular system?

A. Gallamine
B. Pancuronium
C. Tubocurarine
D. Atracurium (Correct Answer)

Explanation: **Explanation:** The cardiovascular stability of a neuromuscular blocking agent (NMBA) depends on its tendency to cause histamine release or its interaction with autonomic receptors (muscarinic or nicotinic). **Why Atracurium is the correct answer:** Atracurium is a benzylisoquinolone derivative known for its **minimal cardiovascular effects** at therapeutic doses. While it can cause some histamine release at very high doses (leading to transient hypotension or flushing), it lacks significant vagolytic or sympathomimetic activity. Furthermore, its unique metabolism via **Hofmann elimination** (spontaneous non-enzymatic degradation) and ester hydrolysis makes it independent of renal or hepatic function, further contributing to its safe profile in diverse clinical scenarios. **Analysis of Incorrect Options:** * **Gallamine:** This is the most cardiotoxic among the options. It has potent **antimuscarinic (vagolytic)** properties, leading to significant tachycardia and hypertension. * **Pancuronium:** An aminosteroid NMBA that exhibits moderate **vagolytic activity** and stimulates the sympathetic nervous system by inhibiting norepinephrine reuptake. This frequently results in tachycardia and increased blood pressure. * **Tubocurarine:** The prototype NMBA, it is notorious for causing significant **histamine release** and **ganglionic blockade**, which often leads to profound hypotension and bronchospasm. **High-Yield Clinical Pearls for NEET-PG:** * **Vecuronium and Rocuronium:** These are even more cardiovascularly stable than Atracurium (virtually no histamine release). * **Hofmann Elimination:** Atracurium and Cisatracurium are the drugs of choice in **liver and kidney failure**. * **Laudanosine:** A metabolite of Atracurium that can cross the BBB and potentially cause seizures (though rare in clinical practice). * **Cisatracurium:** An isomer of atracurium that is more potent and causes **less histamine release** than atracurium.

Question 139: Which of the following effects is specifically observed with intravenous diazepam and not with other routes of administration?

A. Analgesia
B. Sedation
C. Hypotension
D. Coronary dilatation (Correct Answer)

Explanation: **Explanation:** The correct answer is **Coronary dilatation**. **Why it is correct:** Intravenous (IV) diazepam is formulated in a solvent called **propylene glycol** because diazepam is poorly soluble in water. When administered intravenously, this solvent—rather than the diazepam molecule itself—exerts a direct vasodilatory effect on the coronary arteries. This specific hemodynamic effect is unique to the IV route and is not observed when the drug is administered orally or intramuscularly. **Analysis of incorrect options:** * **A. Analgesia:** Benzodiazepines (BZDs) like diazepam do not possess analgesic properties. They are used for sedation, anxiolysis, and muscle relaxation, but they do not act on opioid receptors or inhibit pain pathways. * **B. Sedation:** Sedation is the primary pharmacological effect of diazepam regardless of the route (Oral, IV, or Rectal). It is not specific to the intravenous route. * **C. Hypotension:** While rapid IV injection of diazepam can cause a transient drop in blood pressure due to peripheral vasodilation (also partly due to propylene glycol), it is less characteristic and less specific than the coronary dilatation effect often highlighted in pharmacological literature. **High-Yield Clinical Pearls for NEET-PG:** * **Propylene Glycol Toxicity:** High doses of IV diazepam or lorazepam can lead to propylene glycol toxicity, manifesting as anion gap metabolic acidosis and acute kidney injury. * **Injection Site:** IV diazepam is notorious for causing **thrombophlebitis** (painful inflammation of the vein) due to the propylene glycol solvent. * **Drug of Choice:** While diazepam was historically the drug of choice for **Status Epilepticus**, current protocols often prefer **Lorazepam** (IV) due to its longer duration of action in the brain. * **Antidote:** The specific antagonist for benzodiazepine overdose is **Flumazenil**.

Question 140: For therapeutic effect in manic depressive illness, what steady-state serum lithium concentration should be maintained?

A. 0.2-0.4 mEq/L
B. 0.5-0.8 mEq/L (Correct Answer)
C. 1.0-1.3 mEq/L
D. 1.5-2.5 mEq/L

Explanation: **Explanation:** Lithium is the gold-standard mood stabilizer for Bipolar Disorder (Manic Depressive Illness). It has a **narrow therapeutic index**, meaning the difference between a therapeutic dose and a toxic dose is very small. Therefore, Therapeutic Drug Monitoring (TDM) is mandatory. 1. **Why 0.5–0.8 mEq/L is correct:** For **maintenance therapy** (prophylaxis) in manic-depressive illness, the target steady-state serum concentration is generally **0.5 to 0.8 mEq/L**. This range is effective in preventing relapses while minimizing long-term side effects. Some guidelines allow up to 1.0 mEq/L for maintenance, but 0.5–0.8 is the standard "sweet spot" for stable patients. 2. **Analysis of Incorrect Options:** * **0.2–0.4 mEq/L:** This is **sub-therapeutic**. Concentrations below 0.5 mEq/L are associated with a significantly higher risk of relapse into mania or depression. * **1.0–1.3 mEq/L:** This range is typically reserved for the **acute manic phase**. Higher levels are needed to control active symptoms, but staying at this level long-term increases the risk of renal and thyroid toxicity. * **1.5–2.5 mEq/L:** This is the **toxic range**. Mild toxicity (tremors, vomiting) starts >1.5 mEq/L; severe toxicity (seizures, coma) occurs >2.0 mEq/L. **High-Yield Clinical Pearls for NEET-PG:** * **Sampling Time:** Serum lithium should be measured **12 hours after the last dose** (trough level). * **Steady State:** It takes approximately **5 days** (5 half-lives) to reach steady-state levels. * **Drug Interactions:** Thiazides, NSAIDs, and ACE inhibitors increase lithium levels (by decreasing renal clearance), leading to toxicity. * **Side Effects:** L-I-T-H: **L**eukocytosis, **I**nsipidus (Nephrogenic Diabetes Insipidus), **T**remors/Teratogenicity (Ebstein's Anomaly), **H**ypothyroidism.

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