Bladder Diverticulum

A 72-year-old man with long-standing benign prostatic hyperplasia and recurrent urinary tract infections (3 episodes in 6 months) undergoes bladder ultrasound. The scan shows a well-distended bladder with trabeculated walls and multiple outpouchings. The structure marked **A** in the diagram represents a thin-walled outpouching from the lateral bladder wall measuring 4.5 cm, containing anechoic urine with low-level internal echoes. Post-void residual urine is noted both in the main bladder (200 mL) and within this outpouching (40 mL). Which of the following best explains why this structure marked **A** retains urine and predisposes to recurrent infections?