A 48-year-old man presents to the endocrinology clinic with progressive changes in his facial appearance over the past 8 years. His wife notes that his face has become "coarser" and his jaw has protruded significantly. On examination, the feature marked **B** in the diagram is evident, along with dental malocclusion and increased spacing between his teeth. Serum IGF-1 is markedly elevated, and an oral glucose tolerance test shows failure of growth hormone to suppress below 1 ng/mL. MRI sella reveals a 2.5 cm pituitary adenoma with suprasellar extension. Which of the following best explains the pathophysiology of the clinical finding marked **B**?