Question category: Biochemistry
A 62 year old gentleman with type 2 diabetes mellitus presents unwell with fever, shortness of breath and a cough productive of green sputum. He is found to have a glucose of 21 mmol/L and a sodium of 129 mmol/L. He is only on metformin normally. His urine osmolality is 350 mmol/Kg and urinary sodium is 30 mmol/L. He appears dehydrated. What is the most likely cause of his hyponatraemia?