What condition does pyelonephritis refer to?

Pyelonephritis refers specifically to an infection of the kidney itself, which includes the renal interstitium and the renal pelvis. This condition typically occurs when bacteria ascend from the lower urinary tract, through the ureters, and into the kidneys, leading to inflammation and potential damage.

The correct choice aligns with the definition of pyelonephritis as it emphasizes the inflammatory nature of the kidney lining, particularly in response to the infection. This inflammation is characterized by symptoms such as flank pain, fever, chills, and urinary symptoms, which directly reflect the kidney's involvement.

The other given options reference different conditions: inflammation of the kidney lining pertains to pyelonephritis, but does not distinguish it from other potential inflammatory processes. Additionally, an infection of the urinary bladder refers to cystitis, which specifically involves the lower urinary tract and does not encompass the kidney. Obstruction in the urinary tract and infection in the prostate are separate urological issues that do not accurately describe pyelonephritis, which is a kidney-specific condition.