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A 71-year-old man is brought to the emergency department by ambulance after he was found by his neighbor walking around his garden confused and unable to answer questions. He is confused and oriented only to person. His temperature is 37.6° C (99.7° F), heart rate is 110/min, respiratory rate is 20/min, and blood pressure is 130/92 mm Hg. While in the emergency department the patient has a seizure. Laboratory tests show a serum sodium level of 115 mEq/L, potassium of 3.8 mEq/L, glucose of 100 mg/dL, and osmolality of 250 mOsm/ kg. Urine electrolyte testing show a urine osmolality of 500 mOsm/kg. X-ray of the chest shows a mass in the lung. Which of the following is the most likely etiology of the mass?

(A) Foreign body aspiration
(B) Metastatic brain cancer
(C) Prostate cancer
(D) Small cell carcinoma
(E) Tuberculosis
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(D) Small cell carcinoma
The correct answer is D. The patient’s presentation is consistent with syndrome of inappropriate ADH secretion (low serum osmolality and sodium, with high urine osmolarity), which can be caused by the secretion of ADH from small cell carcinomas of the lung. This type of carcinoma is clearly associated with cigarette smoking.

Answer A is incorrect. An aspirated foreign body can be associated with confusion if it is associated with a decreased gag reflex. However, it is unlikely to cause the electrolyte ab- normalities seen in this patient.

Answer B is incorrect. Brain cancer could cause the altered mental status, but would not be associated with the observed electrolyte abnormalities.

Answer C is incorrect. Prostate cancer is most likely to metastasize to the bone, and is unlikely to cause the electrolyte abnormalities seen in this patient.

Answer E is incorrect. TB would not cause the electrolyte abnormality seen in this patient.