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MDMA Complications — MRCPsych Paper B MCQ

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HardSubstance MisuseMDMA ComplicationsMRCPsych Paper B

A 25-year-old man is brought to the emergency department 3 hours after taking MDMA at a nightclub. He reportedly drank approximately 4 litres of water. Initially, his temperature is 40.2°C and he has sinus tachycardia, confusion, bruxism and dilated pupils. Following active cooling, his temperature falls to 38.4°C, but he becomes increasingly drowsy and vomits. There is no clonus, hyperreflexia or generalised rigidity. Serum sodium is 118 mmol/L, measured serum osmolality is 250 mOsm/kg, creatine kinase is 420 U/L and alanine aminotransferase is normal. Which acute complication best explains his neurological deterioration and carries the greatest immediate neurological risk?

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Correct answer: D — Acute hyponatraemic cerebral oedema

The correct answer is D. This is acute hyponatraemic cerebral oedema: MDMA can increase vasopressin-mediated water retention and thirst, while excessive hypotonic fluid intake further lowers serum sodium. Acute hypo-osmolar hyponatraemia at 118 mmol/L causes water to enter brain cells, producing vomiting, worsening encephalopathy, seizures and potentially fatal cerebral herniation. Deterioration despite successful cooling supports hyponatraemia rather than hyperthermia as the principal neurological driver. Serotonin toxicity is less likely without clonus, hyperreflexia or generalised rigidity. The modest creatine kinase and absence of renal findings do not support clinically significant rhabdomyolysis. Sinus tachycardia is not a ventricular arrhythmia, and normal alanine aminotransferase makes acute hepatic failure unlikely.

Reference: Garcia MR, Gomes NGM, Dias-da-Silva D. Rare but relevant: MDMA and hyponatraemia. Addiction. 2026;121(3):713-718. https://pubmed.ncbi.nlm.nih.gov/41360080/