Question 162

Question:

A 74-year-old man has had confusion for 2 weeks. He has smoked two packs of cigarettes daily for 50 years. An x-ray of the chest shows a 5-cm mass in the lung. Laboratory studies of serum are above. Which of the following is the most likely cause of these findings?

Correct Answer: E.  Small cell carcinoma of the lung

The first thing you need to recognize to answer this question is that this patient has hyponatremia (110 mEq/L, normal is between 135-145). Given the clinical finding of a lung mass, the most likely explanation is that this patient has SIADH, a paraneoplastic complication of small cell carcinoma of the lung (choice E). SIADH is syndrome of inappropriate antidiuretic hormone (ADH) secretion, in which the body, or in this case the lung tumor, secretes ADH that is not responsive to normal feedback mechanisms, resulting in inappropriate retention of water and dilution of the sodium concentration.
The other malignancies in the answer choices are not associated with SIADH. Adenocarcinoma of the lung (choice A) is more common in non-smokers and is not associated with SIADH.
Craniopharyngioma (choice B) is usually a tumor of childhood that is a remnant of Rathke’s pouch; it could compress the pituitary but this would lead to decreased ADH production, not increased.
Medullary carcinoma of the thyroid (choice C) could secrete excess calcitonin, but not ADH.
Renal cell carcinoma (choice D) can secrete excess erythropoietin, leading to polycythemia, or excess red blood cells, but this would have no effect on sodium concentration.