MRCP2-1624

A 65-year-old woman presents a week after experiencing Campylobacter gastroenteritis with increased thirst and frequent urination. She reports drinking 9-10 glasses of water or tea a day, which is twice her normal intake, and her urine is clear. She also experiences fatigue and occasional leg cramps. On examination, she appears euvolaemic with moist mucosa and no oedema. Her blood pressure is 105/90 mmHg and heart rate is 67/min. Lab results show low potassium levels and a pending water deprivation test. What is the most likely cause of her symptoms?