Acute focal bacterial pyelonephritis.

Acute focal bacterial pyelonephritis is a renal inflammatory disease that has similarities to both pyelonephritis and renal abscess. The diagnosis is based on clinical symptoms of pyelonephritis and renal abnormalities detected on radiologic imaging studies (ultrasonography and computed tomographic scanning). Ultrasonographic examination demonstrates mass lesions in the renal cortex that resolve after appropriate antibiotic therapy. Computed tomographic studies reveal localized, wedge-shaped or circular, poorly enhancing, hypodense areas and/or swelling of the superior pole of the kidney. A voiding cystourethrogram should be done to rule out reflux as an underlying cause. Magnetic resonance imaging is not required for diagnosis or follow-up evaluation. Escherichia coli is the most common etiologic agent. All reported cases have responded to conservative therapy with extended courses of oral bactericidal antibiotics. Resolution is typically complete in one to three months. A follow-up evaluation with ultrasonography is required to document resolution.