A 45-year-old woman with a 20-year history of analgesic abuse (chronic NSAID use for migraine) presents with flank pain and hematuria. An intravenous urogram (IVU) 5-minute film is obtained. The structure marked **A** (minor calyx) shows loss of its normal sharp, cup-shaped appearance with flattening and blunting of the fornix. The overlying renal cortex demonstrates focal scarring. Which of the following best explains these radiological findings?