A 32-year-old woman presents to the emergency department with a 3-day history of fever (39.5°C), left flank pain, and dysuria. She has a history of recurrent urinary tract infections. On examination, she has left costovertebral angle (CVA) tenderness. Urinalysis shows pyuria, bacteriuria, and WBC casts. Urine culture grows E. coli. Non-contrast CT abdomen shows left renal swelling with striated nephrogram and perinephric fat stranding. What is the most likely histopathological finding in the affected kidney?

Explanation

## Acute Pyelonephritis: Histopathology ### Clinical Presentation Recognition This patient presents with the classic triad of acute pyelonephritis: fever, flank pain with CVA tenderness, and pyuria with WBC casts. The imaging findings (renal swelling, striated nephrogram, perinephric fat stranding) confirm acute pyelonephritis. ### Histopathological Features of Acute Pyelonephritis **Key Point:** Acute pyelonephritis is characterized by acute suppurative (purulent) inflammation of the renal interstitium and tubules. The hallmark microscopic findings include: 1. **Acute inflammatory infiltrate** — predominantly neutrophils (PMNs) 2. **Location** — interstitium and tubular lumens (not glomeruli) 3. **Tubular involvement** — neutrophils fill tubular lumens, creating "abscess-like" collections 4. **Interstitial edema** — due to acute inflammation 5. **Preservation of glomeruli** — glomeruli are typically spared in uncomplicated pyelonephritis **High-Yield:** The inflammation is **interstitial and tubular**, NOT glomerular. This distinguishes it from glomerulonephritis. ### Comparison with Other Renal Pathologies | Finding | Acute Pyelonephritis | Chronic Pyelonephritis | Acute Glomerulonephritis | Acute Tubular Necrosis | | --- | --- | --- | --- | --- | | **Primary location** | Interstitium + tubules | Interstitium | Glomeruli | Tubular epithelium | | **Inflammation type** | Acute (neutrophils) | Chronic (lymphocytes, fibrosis) | Immune-mediated | Non-inflammatory | | **WBC casts** | Present | Absent | RBC casts present | Muddy brown casts | | **Glomerular involvement** | Spared | Spared | Primary involvement | Spared | | **Tubular atrophy** | Absent | Present | Absent | Epithelial flattening | **Clinical Pearl:** The presence of **WBC casts** (not RBC casts) is pathognomonic for pyelonephritis and indicates tubular involvement with inflammation. ### Why This Patient's Findings Match Acute Pyelonephritis - Positive urine culture (bacterial infection) - WBC casts (tubular inflammation) - Pyuria (neutrophilic response) - Acute presentation (3 days) - Imaging shows acute renal swelling (edema from acute inflammation) [cite:Robbins 10e Ch 20]