A patient is brought to the emergency room following a seizure. Serum electrolyte studies demonstrate serum sodium of 128 mEq/L. The urine osmolarity is higher than the serum osmolarity. Chest x-ray demonstrates a lung mass. Which of the following forms of lung cancer is MOST likely to cause the described electrolyte imbalance?

The patient has SIADH (syndrome of inappropriate antidiuretic hormone secretion), which can be caused by ectopic ADH secretion by small cell carcinomas of the lung, CNS disorders, chronic pulmonary disease, and ceain drugs. Features of SIADH include excessive water retention, hyponatremia (which can lead to seizures when severe), and serum hypo-osmolarity with urine osmolarity greater than serum osmolarity. Associate adenocarcinoma with peripheral lung cancer and lung cancer arising in scars. Associate bronchioloalveolar carcinoma with alveolar-like spaces and no link to smoking. Large cell carcinoma is an aggressive, undifferentiated lung neoplasm. Ref: Else T., Hammer G.D. (2010). Chapter 19. Disorders of the Hypothalamus & Pituitary Gland. In S.J. McPhee, G.D. Hammer (Eds), Pathophysiology of Disease, 6e.