USMLE PRACTICE QUESTION OF THE WEEK

Question for October 17, 2025

A 63-year-old man is brought to the emergency department for progressive confusion. He has a history of hypertension and began hydrochlorothiazide 3 weeks ago. He does not drink alcohol. He is somnolent but arousable. Temperature is 36.9°C (98.4°F), BP 136/82 mm Hg, HR 74/min. Exam shows no focal neurologic deficits.

Laboratory results:

Sodium: 117 mEq/L
Serum osmolality: 254 mOsm/kg
Urine sodium: 48 mEq/L
Urine osmolality: 630 mOsm/kg

Which of the following best explains this patient’s electrolyte abnormality?

A. Syndrome of inappropriate ADH secretion

B. Thiazide-induced hyponatremia

C. Primary polydipsia

D. Adrenal insufficiency

E. Diabetes insipidus

Get Detailed Explanation

Correct Answer:

B. Thiazide-induced hyponatremia

Correct Answer Explanation:

The patient's severe hypotonic hyponatremia with concentrated urine after recent thiazide initiation indicates thiazide-induced hyponatremia. SIADH produces similar labs but is linked to CNS or pulmonary disease. Primary polydipsia causes dilute urine. Adrenal insufficiency would cause hypotension and hyperkalemia. Diabetes insipidus presents with hypernatremia and dilute urine.

Further Insight:

Thiazides impair urinary dilution at the distal tubule.
Loop diuretics more often cause hypokalemia than hyponatremia.
Severe hyponatremia can lead to seizures; correction must be gradual.