CNS Pharmacology: MRCEM Primary questions with answers

CNS Pharmacology covers antipsychotics, antiemetics, antiepileptics, lithium, tricyclic antidepressants and analgesics, as well as the drug management of status epilepticus. Pharmacology carries 24 of the 180 questions in MRCEM Primary. Seizures, agitation, vomiting, pain and overdose are all routine in emergency practice, so knowing how these drugs work, and how they cause harm, is directly useful at the bedside.

5 free questions below, each with the answer and the full explanation · 81 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · CNS Pharmacology

   A male aged 22 is brought to the emergency department by his friends after taking a high dose of amphetamines at a party. He appears agitated, has a rapid heart rate, and is sweating profusely. Which of the following signs is most likely to be present as a direct effect of amphetamine intoxication?

   1. ADilated pupils
   2. BDecreased respiratory rate
   3. CHypotension
   4. DBradycardia
   5. EConstricted pupils
   Show answer and explanation

   Answer: A. Amphetamines are sympathomimetic, directly causing mydriasis alongside tachycardia, sweating and agitation.

   • B: Sympathomimetics tend to raise respiratory rate.
   • C: Hypertension, not hypotension, is expected.
   • D: Tachycardia, not bradycardia, is expected.
   • E: Constricted pupils suggest opioids or cholinergics.

   Full explanation

   Toxidromes (High Yield):

2. Question 2 · Analgesics

   A male patient aged 30 is brought into ED unresponsive; after initial assessment you suspect opioid toxicity and start naloxone. On which one of the following receptors does naloxone work?

   1. ANMDA receptor Antagonist
   2. BOpioid mu receptors antagonist
   3. CNMDA receptor agonist
   4. DOpioid mu receptors agonist
   5. EGaba receptor
   Show answer and explanation

   Answer: B. Naloxone is an opioid mu receptor antagonist that rapidly reverses opioid toxicity.

   • A: NMDA antagonism is the action of ketamine.
   • C: Naloxone is an antagonist, not an agonist.
   • D: Mu agonism is the action of opioids, which naloxone reverses.
   • E: GABA receptors are the target of benzodiazepines.

   Full explanation

   Naloxone is opioid mu receptor antagonist rapidly reverse the symptoms of opioid toxicity.

3. Question 3 · Anti-epileptic drugs

   Which of the following best describes the way lorazepam acts?

   1. AGABA antagonist
   2. BMuscarinic receptors agonist
   3. CPotentiate GABA release
   4. DCa channel blocker
   5. EStimulate acetylcholine release
   Show answer and explanation

   Answer: C. Lorazepam potentiates the inhibitory effect of GABA at GABA-A receptors.

   • A: It enhances rather than antagonises GABA.
   • B: It does not act on muscarinic receptors.
   • D: It does not block calcium channels.
   • E: It does not stimulate acetylcholine release.

   Full explanation

   • Benzodiazepines e.g.: lorazepam is gamma amino butyric acid (GABA) receptors agonist
   • GABA is the major inhibitory neurotransmitter in the central nervous system...
   • Lorazepam overdose can be treated with a specific benzodiazepine antidote medication known as flumazenil.

4. Question 4 · Tricyclic-antidepressants

   A young girl aged 18 years was brought into the ED with severe respiratory depression. Her mum reports that she has finished a packet of her sleeping pills, and you suspect benzodiazepines overdose. Which one of the following drugs will reverse the effect?

   1. ANaloxone
   2. BDoxepin
   3. CRamelteon
   4. DDiazepam
   5. EFlumazenil
   Show answer and explanation

   Answer: E. Flumazenil antagonises benzodiazepine receptors, reversing benzodiazepine effects.

   • A: Naloxone reverses opioids.
   • B: Doxepin is a tricyclic antidepressant.
   • C: Ramelteon is a melatonin agonist hypnotic.
   • D: Diazepam is itself a benzodiazepine.

   Full explanation

   flumazenil is indicated to reverse the effects of benzodiazepines via antagonizing the Benzodiazepines receptor.

   ANTIDOTES: (High yield)

   Benzodiazepines = flumazenil

   Lidocaine: Intralipid emulsion

   Prilocaine: methylene blue

   Tricyclic antidepressants: sodium bicarbonate

   Metoclopramide = procyclidine

   Opioid = naloxone

   Organophosphate= atropine

   Paracetamol poisoning= N-acetyl cysteine

   Hyperkalemia = Ca gluconate

5. Question 5 · Anti-Psychotic drugs

   A man aged 32 with schizophrenia is brought to the emergency department with high-grade fever, generalized muscle rigidity, confusion, and sweating. His blood pressure is fluctuating, and his heart rate is elevated. He was started on haloperidol 3 days ago for agitation. Laboratory tests reveal an elevated creatine kinase (CK) level. What is the most likely diagnosis?

   1. ASerotonin syndrome
   2. BMalignant hyperthermia
   3. CAcute dystonic reaction
   4. DNeuroleptic malignant syndrome
   5. EAnticholinergic toxicity
   Show answer and explanation

   Answer: D. Fever, lead-pipe rigidity, autonomic instability, confusion and raised CK after starting haloperidol indicate neuroleptic malignant syndrome.

   • A: Features clonus, hyperreflexia and diarrhoea after serotonergic drugs; onset within hours.
   • B: Triggered by anaesthetic agents, not antipsychotics.
   • C: Causes focal muscle spasm without fever or autonomic instability.
   • E: Causes dry skin, dilated pupils and urinary retention, not sweating or rigidity.

   Full explanation

   Neuroleptic Malignant Syndrome (NMS) is a rare but serious side effect of antipsychotic drugs like haloperidol.

   Key features include:

   • Fever
   • Severe "lead-pipe" muscle rigidity
   • Autonomic instability (fluctuating BP, tachycardia, sweating)
   • Altered mental status
   • Elevated CK (due to muscle breakdown)