A 52-year-old man presents to the endocrinology clinic with progressive difficulty wearing his wedding ring and shoes over the past 5 years. On examination, the structure marked **A** in the diagram is noted to be broad and spade-like. His wife reports he snores loudly and has episodes of apnea at night. Serum IGF-1 is elevated at 650 ng/mL (normal <150). Which of the following is the most likely underlying pathology?