A 47-year-old man is brought to the ED. His family reports that he had become confused over the past 72 hours and suffered a seizure while on the way to the hospital. Upon arrival, the patient is minimally responsive. His respiratory rate is 32 breaths/minute, pulse rate 110 beats/minute, and blood pressure 120/90 mmHg. Immediate laboratory work reveals an abnormal sodium level of 117 mEq/L. The patient’s chest X-ray reveals a mass in the right mid-lung field. The nurse suspects that the underlying factor responsible for this patient’s change in mental status is related to: