GPAT practice: Pharmacology (ANS and CNS)
20 original GPAT-style questions on autonomic and central nervous system pharmacology and basic pharmacokinetics, with answers and explanations. Tap an option to check your answer, or open "Show answer".
1. Which receptor subtype mediates contraction of the radial muscle of the iris, causing mydriasis?
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Answer: A — Sympathetic stimulation of alpha-1 receptors contracts the radial (dilator) muscle and dilates the pupil. Muscarinic M3 contracts the sphincter pupillae and causes miosis.
2. Neostigmine differs from physostigmine mainly because neostigmine
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Answer: B — Neostigmine carries a permanent positive charge, so it hardly enters the brain, whereas physostigmine is a tertiary amine that does. Both are reversible carbamate cholinesterase inhibitors.
3. Which drug is a direct-acting muscarinic agonist that is resistant to hydrolysis by cholinesterase?
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Answer: C — Pilocarpine is an alkaloid that directly stimulates muscarinic receptors and is not a substrate of acetylcholinesterase. Acetylcholine is rapidly hydrolysed and atropine is an antagonist.
4. Which drug is the preferred first-line agent for anaphylactic shock?
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Answer: D — Adrenaline acts on alpha-1 (vasoconstriction), beta-1 (cardiac output) and beta-2 (bronchodilation), reversing the main features of anaphylaxis. Salbutamol and phenylephrine each cover only part of the picture.
5. Which beta blocker is cardioselective (beta-1) and lacks intrinsic sympathomimetic activity?
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Answer: C — Atenolol is beta-1 selective. Propranolol and timolol are non-selective, and pindolol is non-selective with intrinsic sympathomimetic activity.
6. Atropine poisoning is characterised by which of the following?
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Answer: B — Blocking muscarinic receptors removes parasympathetic tone, causing dry mouth, dilated pupils, tachycardia and loss of sweating (hot, dry skin). Options A and C describe cholinergic excess.
7. Tubocurarine produces skeletal muscle relaxation by
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Answer: C — Tubocurarine is a non-depolarising agent that competitively blocks NM receptors, and its effect can be reversed by neostigmine. Suxamethonium is the depolarising agent.
8. Which action explains the anxiolytic and sedative effects of benzodiazepines?
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Answer: B — Benzodiazepines bind at the interface of GABA-A subunits and raise the frequency of channel opening only when GABA is present. Barbiturates, in contrast, increase the duration of opening and can act directly at high doses.
9. Which drug is a specific competitive antagonist at the benzodiazepine site and is used to reverse benzodiazepine overdose?
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Answer: C — Flumazenil blocks the benzodiazepine binding site on GABA-A receptors. Naloxone is an opioid antagonist and zolpidem is a hypnotic agonist at that site.
10. Which antiepileptic is the drug of choice for absence seizures in a child who has no other seizure type?
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Answer: C — Ethosuximide blocks T-type calcium channels in thalamic neurons, which underlie absence seizures. Phenytoin and carbamazepine can worsen absence seizures.
11. Which statement about phenytoin pharmacokinetics is correct?
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Answer: B — Phenytoin metabolism saturates at therapeutic concentrations, so a small dose increase can cause a disproportionate rise in plasma level. It is mostly hepatically metabolised and is an enzyme inducer.
12. Which antiepileptic is associated with gingival hyperplasia and hirsutism?
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Answer: D — Gingival hyperplasia, hirsutism and coarse facies are characteristic long-term effects of phenytoin. Valproate is better remembered for hepatotoxicity, tremor and neural tube defects.
13. Which antidepressant class acts by irreversibly inhibiting an enzyme that degrades monoamines, with a risk of hypertensive crisis after tyramine-rich food?
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Answer: C — Non-selective irreversible MAO inhibitors such as phenelzine allow tyramine to release large amounts of noradrenaline, causing the "cheese reaction". SSRIs and tricyclics act on reuptake transporters.
14. Which adverse effect is most typical of tricyclic antidepressants such as amitriptyline?
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Answer: D — Tricyclics block muscarinic, histamine H1 and alpha-1 receptors in addition to reuptake, producing anticholinergic effects, sedation and postural hypotension.
15. Serotonin syndrome is most likely when an SSRI is combined with
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Answer: B — Increased synaptic serotonin from reuptake blockade plus reduced breakdown by MAO can produce hyperthermia, agitation, clonus and autonomic instability. The combination is contraindicated.
16. Haloperidol produces its antipsychotic effect mainly through blockade of
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Answer: D — D2 antagonism in the mesolimbic pathway reduces positive symptoms, and the same action in the nigrostriatal pathway explains extrapyramidal side effects. Clozapine has comparatively weak D2 blockade.
17. Which antipsychotic is associated with agranulocytosis and therefore needs blood count monitoring?
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Answer: C — Clozapine carries a recognised risk of agranulocytosis and requires regular white cell monitoring. It has a low incidence of extrapyramidal effects, which is why it is kept for resistant cases.
18. Morphine causes miosis mainly through action on which receptor?
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Answer: C — Pinpoint pupils result from mu-receptor stimulation of the Edinger-Westphal nucleus and are a useful sign in opioid overdose. Little tolerance develops to miosis.
19. A drug has a volume of distribution of 40 L and a clearance of 4 L/h. What is its approximate elimination half-life? (Use t1/2 = 0.693 x Vd / CL)
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Answer: B — t1/2 = 0.693 x 40 / 4 = 6.93 hours. Half-life increases with volume of distribution and decreases with clearance.
20. Oral bioavailability of a drug with extensive first-pass metabolism can be improved by which route?
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Answer: A — Sublingual absorption drains into systemic veins and bypasses the portal circulation and liver, avoiding first-pass loss (as with glyceryl trinitrate). Enteric coating protects from gastric acid but does not avoid hepatic first-pass.