USMLE Step 1 Question of the Day

Practice USMLE Step 1 with the production-style question-of-the-day selection for this public URL.

Question 1

A 62-year-old man with known small cell lung carcinoma presents with headaches and confusion. He appears euvolemic on examination without edema or orthostasis. Labs: sodium 118 mmol/L (135-145), potassium 4.0 mmol/L (3.5-5.0), chloride 90 mmol/L (98-106), bicarbonate 24 mmol/L (22-28), BUN 6 mg/dL (7-20), creatinine 0.8 mg/dL (0.6-1.3), serum osmolality 255 mOsm/kg (275-295), urine osmolality 520 mOsm/kg (50-1200), urine sodium 48 mmol/L, uric acid 2.5 mg/dL (3.5-7.2). TSH 2.1 uIU/mL (0.4-4.0) and morning cortisol 15 mcg/dL (10-20). CT shows a central lung mass. Which of the following best explains these lab findings?

Which of the following best explains these lab findings?

Primary polydipsia causing water overload with maximally dilute urine due to suppressed antidiuretic hormone
Hyperglycemia-mediated translocational hyponatremia with elevated serum osmolality
Loop diuretic–induced volume depletion causing secondary antidiuretic hormone release with low urine sodium
Primary adrenal insufficiency causing mineralocorticoid deficiency with hyperkalemic hyponatremia
Ectopic antidiuretic hormone increasing V2 receptor–mediated aquaporin-2 insertion and free water reabsorption in the collecting duct

Explanation: Euvolemic hyponatremia with low serum osmolality, inappropriately concentrated urine, and high urine sodium is consistent with SIADH, commonly from small cell lung carcinoma. Primary polydipsia would show very low urine osmolality, and hyperglycemia would increase serum osmolality, neither seen here. Diuretic-induced hypovolemia typically has low urine sodium, and primary adrenal insufficiency causes hyperkalemia and hypotension.

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USMLE Step 1

USMLE Step 1 Question of the Day

Practice USMLE Step 1 with the production-style question-of-the-day selection for this public URL.

Question 1

A 62-year-old man with known small cell lung carcinoma presents with headaches and confusion. He appears euvolemic on examination without edema or orthostasis. Labs: sodium 118 mmol/L (135-145), potassium 4.0 mmol/L (3.5-5.0), chloride 90 mmol/L (98-106), bicarbonate 24 mmol/L (22-28), BUN 6 mg/dL (7-20), creatinine 0.8 mg/dL (0.6-1.3), serum osmolality 255 mOsm/kg (275-295), urine osmolality 520 mOsm/kg (50-1200), urine sodium 48 mmol/L, uric acid 2.5 mg/dL (3.5-7.2). TSH 2.1 uIU/mL (0.4-4.0) and morning cortisol 15 mcg/dL (10-20). CT shows a central lung mass. Which of the following best explains these lab findings?

Which of the following best explains these lab findings?

Primary polydipsia causing water overload with maximally dilute urine due to suppressed antidiuretic hormone
Hyperglycemia-mediated translocational hyponatremia with elevated serum osmolality
Loop diuretic–induced volume depletion causing secondary antidiuretic hormone release with low urine sodium
Primary adrenal insufficiency causing mineralocorticoid deficiency with hyperkalemic hyponatremia
Ectopic antidiuretic hormone increasing V2 receptor–mediated aquaporin-2 insertion and free water reabsorption in the collecting duct

Explanation: Euvolemic hyponatremia with low serum osmolality, inappropriately concentrated urine, and high urine sodium is consistent with SIADH, commonly from small cell lung carcinoma. Primary polydipsia would show very low urine osmolality, and hyperglycemia would increase serum osmolality, neither seen here. Diuretic-induced hypovolemia typically has low urine sodium, and primary adrenal insufficiency causes hyperkalemia and hypotension.