This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney, Subject name: Medicine
A 77-year-old man with a mass in the lung develops asymptomatic hyponatremia. His JVP is 4 cm, heart sounds are normal, and the lungs are clear. The urine sodium is 64 mEq/L and osmolality 550 mOsm/kg. Which of the following is the most likely diagnosis? 
Select and explain. 
A. nephrotic syndrome, B. syndrome of inappropriate antidiuretic hormone (SIADH) production, C. renal metastases from lung cancer, D. lung metastases from hypernephroma
B: syndrome of inappropriate antidiuretic hormone (SIADH) production
Answer: B: syndrome of inappropriate antidiuretic hormone (SIADH) production 
Explanation: The urine osmolality in patients with SIADH need not be hypertonic to plasma, but only inappropriately high compared with serum. The major characteristics of SIADH include hyponatremia, volume expansion without edema, natriuresis, hypouricemia, and normal or reduced serum creatinine level, with normal thyroid and adrenal function.