A 46-year-old obese multiparous woman with type 2 diabetes presents with recurrent postprandial right upper quadrant pain radiating to the right shoulder, worse after fatty meals. Ultrasound shows multiple mobile stones and a thickened gallbladder wall measuring 5 mm. Laparoscopic cholecystectomy is performed. The gross specimen shows the structure marked **A** is diffusely thickened (4–6 mm), greyish-white, and firm. Microscopy reveals chronic mononuclear infiltration, fibrosis, and smooth muscle hypertrophy. Which of the following best describes the pathological basis of the wall thickening in chronic cholecystitis?