A 28-year-old man with chronic kidney disease is undergoing renal biopsy. Electron microscopy shows loss of the brush border in tubular epithelial cells with preserved basement membrane. Which segment is most likely affected, and what is the primary functional consequence?

Explanation

## Brush Border Loss and Tubular Dysfunction ### Pathophysiology of Brush Border Damage **Key Point:** The brush border (apical microvilli) is unique to the proximal convoluted tubule and is essential for the massive reabsorptive capacity of this segment. Loss of brush border indicates proximal tubular injury and results in impaired reabsorption of glucose, amino acids, and other filtered solutes. ### Segment-Specific Structural Features | Nephron Segment | Brush Border | Mitochondria | Primary Function | |-----------------|--------------|--------------|------------------| | **PCT** | **Present (tall, dense)** | Very high | Bulk reabsorption of glucose, amino acids, ions, water | | **Loop of Henle** | Absent | Moderate | Countercurrent multiplication (water/ion separation) | | **DCT** | Absent or sparse | Moderate | Selective ion reabsorption (aldosterone-responsive) | | **Collecting Duct** | Absent | Moderate | Water reabsorption (ADH-responsive) | ### Why PCT Loss Causes Glycosuria and Aminoaciduria 1. **Structural basis:** The brush border provides 300–400× increase in apical surface area, allowing simultaneous operation of glucose cotransporters (SGLT1) and amino acid transporters. 2. **Functional consequence:** Without microvilli, filtered glucose and amino acids cannot be reabsorbed efficiently and are lost in urine (glycosuria, aminoaciduria). 3. **Clinical correlation:** This pattern is seen in acute tubular necrosis (ATN), heavy metal poisoning (mercury, lead), and some inherited tubulopathies. **High-Yield:** Glycosuria in the setting of normal blood glucose is pathognomonic for proximal tubular dysfunction. The glomerular filtration barrier is intact (preserved basement membrane on EM), but the tubular reabsorptive machinery is damaged. **Clinical Pearl:** In Fanconi syndrome, widespread proximal tubular dysfunction causes glycosuria, phosphaturia, aminoaciduria, and renal tubular acidosis—all due to loss of proximal tubular epithelial integrity and brush border damage. **Mnemonic:** **BAGEL** — Bicarbonate, Amino acids, Glucose, Electrolytes, Lactate — all reabsorbed in the PCT and lost when brush border is damaged. ### Why Other Segments Are Not Affected - **Loop of Henle:** Has no brush border normally; injury would impair countercurrent multiplication, not glucose reabsorption. - **DCT:** Has sparse or no brush border; injury would impair K⁺ and Na⁺ regulation, not glucose reabsorption. - **Collecting Duct:** Has no brush border; injury would impair water reabsorption (polyuria), not cause glycosuria. [cite:Robbins & Cotran Pathologic Basis of Disease 10e Ch 20]