uk-dgm MCQ (SBA): SIADH Diagnosis - Biochemical Interpretation

A 81-year-old woman presents with acute confusion. She has a background of well-controlled hypertension on Amlodipine 10 mg. She is afebrile. Bloods show Na 119 mmol/L, K 4.8 mmol/L, Urea 3.2 mmol/L, Creatinine 68 µmol/L, serum osmolality 248 mOsm/kg, urine osmolality 520 mOsm/kg, urine sodium 45 mmol/L. What is the MOST likely diagnosis?