"Mehlman High Yield Risk Factors Practice Test 2026 – Your All-in-One Guide to Mastering Risk Factors!"

When hyponatremia occurs with an inappropriately concentrated urine, think SIADH—syndrome of inappropriate antidiuretic hormone secretion. In SIADH, ADH keeps the kidneys reabsorbing water even though the serum is diluted, so you see a low serum osmolality with concentrated urine and high urine sodium, all while the patient often appears euvolemic.

In an older patient with recent weight loss, a paraneoplastic source is a key consideration. The cancer most classically associated with ectopic ADH production is small cell lung cancer. That link explains both the hyponatremia and the concentrated urine pattern here, and the weight loss fits a malignancy picture. The strongest risk factor for this tumor is smoking, which makes small cell lung cancer the most likely culprit.

Other conditions like adrenal insufficiency or hypothyroidism can cause hyponatremia, but they don’t typically present with the same inappropriately concentrated urine pattern in this clinical vignette, and primary polydipsia would usually produce dilute urine rather than concentrated urine.