Pyelonephritis

Which of the following is the most common causative organism of acute pyelonephritis in non-pregnant, non-catheterized women?

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?

A 28-year-old woman with a 2-week history of recurrent pyelonephritis (treated with antibiotics) presents with persistent fever, flank pain, and imaging showing a 4 cm fluid collection in the left perinephric space with gas locules. Blood cultures are positive for E. coli. What is the most likely complication and its underlying pathological mechanism?

Which histopathological finding is characteristic of acute pyelonephritis on light microscopy?