Lateral Skull X-Ray Acromegaly

A 48-year-old man presents to the endocrinology clinic with progressive coarsening of facial features, jaw enlargement, and ring tightening over the past 8 years. A lateral skull X-ray is obtained. The structure marked **A** in the diagram shows enlargement to 13 mm in depth. Serum IGF-1 is elevated at 2.5× the upper limit of normal for his age. Growth hormone fails to suppress below 1 ng/mL after a 75 g oral glucose tolerance test. Which of the following best explains the radiological finding at **A** and the biochemical abnormality?