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Lorazepam in catatonia — MRCPsych Paper A MCQ

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HardPsychopharmacologyLorazepam in catatoniaMRCPsych Paper A

A 58-year-old woman with mutism, negativism and posturing has a marked reduction in these signs within 10 minutes of intravenous lorazepam. Which receptor-level action of lorazepam most directly accounts for its pharmacodynamic effect?

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Correct answer: C — Positive allosteric modulation at the benzodiazepine site, increasing the frequency of GABA-evoked chloride-channel opening

Lorazepam is a benzodiazepine and a positive allosteric modulator of GABA-A receptors. It binds at the benzodiazepine site rather than the orthosteric GABA site and, in the presence of GABA, increases the frequency of chloride-channel opening, enhancing inhibitory neurotransmission. This can produce a rapid reduction in catatonic signs. It is not a direct GABA-A agonist, so option B is incorrect. Increasing channel-opening duration is classically associated with barbiturates, not benzodiazepines. GABA-B agonism describes drugs such as baclofen. NMDA receptor antagonism is relevant to agents such as ketamine, memantine or amantadine rather than lorazepam. The precise pathophysiology of catatonia is heterogeneous, but lorazepam's receptor pharmacology is well established.

Reference: Electronic Medicines Compendium, Lorazepam 1 mg Orodispersible tablets, Summary of Product Characteristics, section 5.1 Pharmacodynamic properties, revised 2026. https://www.medicines.org.uk/emc/product/101894/smpc