A 62-year-old man with small-cell lung cancer presents with confusion and lethargy. Serum sodium is 118 mEq/L, serum osmolality is 245 mOsm/kg, and urine osmolality is 580 mOsm/kg. Which of the following is NOT a recognized mechanism of hyponatremia in this clinical scenario?

Explanation

## Clinical Context: SIADH in Malignancy This patient has **Syndrome of Inappropriate Antidiuretic Hormone (SIADH)** — a common paraneoplastic syndrome in small-cell lung cancer (SCLC). ### Diagnostic Criteria Present - Hyponatremia (Na 118 mEq/L) - Low serum osmolality (245 mOsm/kg) — hypotonic - Inappropriately high urine osmolality (580 mOsm/kg) — urine is *more* concentrated than plasma - This combination is pathognomonic for SIADH ### Mechanism of Hyponatremia in SIADH **Key Point:** SIADH causes hyponatremia through **excessive free water reabsorption**, not sodium loss. 1. **Ectopic ADH secretion** — SCLC cells produce ADH (vasopressin), leading to constitutive activation of V2 receptors in the collecting duct 2. **Increased aquaporin-2 insertion** — ADH causes increased water channel expression in collecting duct principal cells 3. **Impaired free water excretion** — kidneys cannot dilute urine despite low serum osmolality; urine remains hyperosmolar 4. **Net free water retention** — dilutes plasma sodium progressively ### Why Each Option Is or Isn't Correct | Mechanism | Status | Explanation | |-----------|--------|-------------| | Ectopic ADH from tumor | ✓ Correct | Direct cause of SIADH in SCLC | | Increased renal free water reabsorption | ✓ Correct | ADH-mediated aquaporin-2 activation | | **Nephrogenic DI from chemotherapy** | ✗ **Wrong** | DI causes *hypernatremia*, not hyponatremia; opposite mechanism | | Impaired dilute urine excretion | ✓ Correct | Kidneys cannot respond to osmotic stimulus | **High-Yield:** Nephrogenic DI (from lithium, amphotericin B, cisplatin) causes **inability to concentrate urine** → polyuria → hypernatremia. This is the *opposite* of SIADH pathophysiology. ### Clinical Pearl **Warning:** Do not confuse SIADH with nephrogenic DI: - **SIADH** → hyponatremia + high urine osmolality (kidneys *over*-concentrate) - **Nephrogenic DI** → hypernatremia + low urine osmolality (kidneys *under*-concentrate) Chemotherapy agents (especially cisplatin) cause nephrogenic DI, which would produce the *opposite* electrolyte picture from what this patient has.