20th June 2023

A 54-year-old woman presents to the emergency department with acute onset confusion and is found to have a sodium of 117

A 54-year-old woman presents to the emergency department with acute onset confusion and is found to have a sodium of 117. Paired serum and urinary sodium and osmolalities are performed, and the results are consistent with syndrome of inappropriate antidiuretic hormone (SIADH). She takes no regular medication, is a lifelong non-smoker, and has a past medical history of primary mediastinal lymphoma in her 20s, treated with radiotherapy and chemotherapy. A CT chest/abdomen/pelvis is performed revealing widespread liver and bone deposits consistent with a metastatic malignancy, without an obvious primary.

What is the most likely underlying malignancy?

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