A 62-year-old man undergoes digital rectal examination (DRE) during a health check-up. A nodular, asymmetric induration is palpated in the posterior aspect of the prostate. Biopsy confirms adenocarcinoma. The structure marked **A** in the diagram represents the zone from which this cancer most likely originated. Which of the following best explains why this zone is the predominant site of malignant transformation in prostate cancer?