A man undergoes a pneumonectomy. After surgery, investigation show hyponatremia. What could be the cause of the biochemical change?

The key is A. Removal of harmonically active tumour. [Ectopic ACTH secreting tumour causes hypernatremia and body's homeostatic mechanism try to lower the level of high sodium and do a lesser degree though sodium remains in hypernatremic level or even it may be normal (this question does not mention any preoperative hypernatremia). Removal of that tumour results in negative sodium balance for time being which results hyponatremia while gradually it tends to rise again to normal level].