Obstructive Hydronephrosis from Ureteric Calculus

A 52-year-old man with recurrent urolithiasis presents with acute severe left flank pain radiating to the groin and ipsilateral testicle, associated with nausea, vomiting, and microscopic haematuria. He is writhing in agony. Serum creatinine is 128 µmol/L (baseline 82 µmol/L). Renal ultrasound shows a dilated left pelvicalyceal system with the structure marked **A** (ballooned calyces) and mildly thinned cortex. Non-contrast CT KUB confirms a 7 mm obstructing calculus at the left vesicoureteric junction with proximal hydroureteronephrosis. The appearance of **A** in this case is a direct consequence of which of the following?