A 28-year-old man from Delhi presents to the nephrology clinic with polyuria (8–10 L/day) and polydipsia. Serum osmolality is 305 mOsm/kg, urine osmolality is 150 mOsm/kg, and plasma ADH is elevated at 15 pg/mL. A water deprivation test shows no change in urine osmolality. Which segment of the nephron is MOST likely to be dysfunctional in this patient?

Explanation

## Clinical Diagnosis: Nephrogenic Diabetes Insipidus (NDI) ### Pathophysiology The patient presents with: - High serum osmolality (305 mOsm/kg) — hyperosmolar state - Low urine osmolality (150 mOsm/kg) — dilute urine despite hyperosmolarity - Elevated ADH (15 pg/mL) — appropriate ADH response to hypernatremia - No response to water deprivation — confirms **nephrogenic** (not central) DI This constellation indicates **resistance of the kidney to ADH**, not deficiency of ADH. ### Nephron Segment Involved **Key Point:** The collecting duct principal cells are the **sole site of ADH-mediated water reabsorption** in the nephron. ADH binds to V2 receptors on the basolateral membrane of principal cells, triggering cAMP production and insertion of aquaporin-2 (AQP2) water channels into the apical membrane. In nephrogenic DI, either: - V2 receptor mutation (X-linked NDI, ~90% of cases) - AQP2 channel mutation (autosomal recessive NDI) - Medullary interstitial disease reducing osmotic gradient Without functional water channels in the collecting duct, the kidney cannot concentrate urine despite high ADH levels. ### Why Other Segments Are Not Responsible | Nephron Segment | Function | Role in Osmolality | Dysfunction Effect | |---|---|---|---| | **Proximal tubule** | Isosmotic reabsorption of glucose, amino acids, Na^+^, water | Maintains isotonicity; reabsorbs 65% of filtered water | Fanconi syndrome, not polyuria with high serum osmolality | | **Thick ascending limb** | Active Na^+^/K^+^/2Cl^−^ transport; **impermeable to water** | Creates positive osmotic gradient in medulla | Dysfunction → salt wasting, not ADH resistance | | **Distal convoluted tubule** | Fine-tuning of Na^+^ reabsorption (aldosterone-sensitive) | Minor water reabsorption; NOT ADH-responsive | Dysfunction → hyponatremia or hyperkalemia, not polyuria | **Clinical Pearl:** The **thick ascending limb is impermeable to water by design** — it generates the medullary osmotic gradient needed for the collecting duct to concentrate urine. Damage here causes salt wasting, not polyuria with high serum osmolality. ### High-Yield Mnemonic **"ADH acts on the Collecting duct"** — the only nephron segment with V2 receptors and aquaporin-2 channels. All other segments lack ADH responsiveness. ### Diagnostic Confirmation - **Water deprivation test:** No rise in urine osmolality → nephrogenic DI - **Desmopressin (synthetic ADH) challenge:** No response in nephrogenic DI (unlike central DI, which responds) - **Genetic testing:** Confirm V2R or AQP2 mutation if suspected hereditary form