A 45-year-old man from rural India presents with progressive fatigue, weight loss, and salt craving over 3 months. On examination, he has marked hyperpigmentation of the **buccal mucosa (A)**, palmar creases, and extensor surfaces. Serum sodium is 128 mEq/L, potassium 5.8 mEq/L, and morning cortisol is 2 μg/dL with ACTH 180 pg/mL. Which of the following best explains the hyperpigmentation seen at the site marked **A**?