MSFinals-6195

MSFinals-6195

A 28-year-old woman is recovering on the ward after experiencing a subarachnoid haemorrhage 6 days ago. She has been able to maintain her oral fluid intake above 3 litres per day and her heart rate is 72 bpm at rest, while her blood pressure is 146/88 mmHg at rest. Over the last 6 days, her fluid balance shows that she is net positive 650 ml. Her daily blood tests reveal the following results:

– Hb 134 g/l
– Platelets 253 * 109/l
– WBC 5.1 * 109/l
– Neuts 3.9 * 109/l
– Lymphs 1.2 * 109/l
– Na+ 129 mmol/l
– K+ 4.1 mmol/l
– Urea 2.3 mmol/l
– Creatinine 49 µmol/l
– CRP 12.3 mg/l

Paired serum and urine samples show the following:

– Serum Osmolality 263 mosm/l
– Urine Osmolality 599 mosm/l
– Serum Na+ 129 mmol/l
– Urine Na+ 63 mmol/l

What is the most likely reason for the patient’s hyponatraemia?