A 32-year-old man from Delhi presents to the nephrology clinic with a 3-month history of polyuria (8–10 L/day) and polydipsia. Serum sodium is 148 mEq/L, serum osmolality 310 mOsm/kg, and urine osmolality 150 mOsm/kg despite high serum osmolality. A water deprivation test shows no improvement in urine osmolality, but intranasal desmopressin (DDAVP) administration results in urine osmolality rising to 600 mOsm/kg. MRI brain reveals a sellar mass compressing the pituitary stalk. Which segment of the nephron is primarily responsible for the concentrating defect seen in this patient?

Explanation

## Clinical Context: Central Diabetes Insipidus This patient has **central diabetes insipidus** (CDI) secondary to pituitary stalk compression. The key diagnostic clue is the **response to exogenous DDAVP**, which confirms ADH deficiency rather than nephrogenic resistance. ## Nephron Segment Responsible for Urine Concentration **Key Point:** The **collecting duct** is the primary site of action of ADH (vasopressin) and is responsible for the final concentration of urine. ### Mechanism of ADH Action in the Collecting Duct 1. ADH binds to V~2~ receptors on the basolateral membrane of principal cells 2. Activates adenylyl cyclase → ↑ cAMP → activation of protein kinase A (PKA) 3. PKA phosphorylates and inserts **aquaporin-2 (AQP2) water channels** into the apical membrane 4. Water reabsorption increases dramatically → concentrated urine 5. In the absence of ADH, AQP2 channels are internalized → dilute urine ### Why the Collecting Duct? | Segment | ADH Sensitivity | Primary Function | Osmolality Change | |---------|-----------------|------------------|-------------------| | **Collecting Duct** | **High** | **Final water reabsorption** | **Concentrated urine (up to 1200 mOsm/kg)** | | Proximal convoluted tubule | No | Bulk reabsorption of glucose, amino acids, NaCl, water | Isotonic | | Thick ascending limb | No | Active NaCl reabsorption (dilution) | Hypotonic fluid | | Distal convoluted tubule | Minimal | Fine-tuning of NaCl; aldosterone-responsive | Hypotonic | **Clinical Pearl:** In CDI, the collecting duct cannot respond to ADH (or ADH is absent), resulting in failure to concentrate urine. Exogenous DDAVP restores the osmotic gradient and allows water reabsorption, confirming the diagnosis. **High-Yield:** The collecting duct is the **only nephron segment whose permeability to water is regulated by ADH**. Without ADH, it remains impermeable; with ADH, it becomes highly permeable. ## Why This Patient's Urine Is Dilute - Pituitary stalk compression → ↓ ADH secretion - Collecting duct principal cells lack phosphorylated AQP2 channels - Water cannot be reabsorbed despite high serum osmolality - Result: polyuria with dilute urine (150 mOsm/kg) - DDAVP restores the pathway → urine concentrates to 600 mOsm/kg [cite:Guyton & Hall Physiology 14e Ch 28]