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 has no relevant past medical history and takes no regular medication, but is a heavy smoker with a 40-pack-year history. A CT chest/abdomen/pelvis is performed revealing widespread liver and bone deposits consistent with a metastatic malignancy, with small volume mediastinal lymphadenopathy but without an obvious primary.

What is the most likely underlying malignancy?

Select the SINGLE most appropriate option from the list below. Select ONE option only.