A 55-year-old woman with acute cholecystitis (confirmed by ultrasound: wall thickening, pericholecystic fluid, positive sonographic Murphy's sign) undergoes conservative management with IV fluids and antibiotics. After 48 hours, her fever persists, WBC remains elevated at 16,000/μL, and repeat ultrasound shows a 4 cm fluid collection adjacent to the gallbladder. A 60-year-old man with similar acute cholecystitis presentation is managed identically; at 48 hours, his fever has resolved, WBC is 10,500/μL, and repeat ultrasound shows resolving wall edema with no fluid collection. Which imaging/clinical finding BEST distinguishes acute suppurative cholecystitis from uncomplicated acute cholecystitis?