A 52-year-old woman with a 10-year history of asymptomatic gallstones is found incidentally on ultrasound during evaluation for dyspepsia. Her liver function tests are normal. In contrast, a 48-year-old man presents with acute right upper quadrant pain, fever (38.5°C), and Murphy's sign positive; ultrasound shows gallstones with gallbladder wall thickening and pericholecystic fluid. Which feature BEST discriminates acute cholecystitis from asymptomatic cholelithiasis?