The presence of free fluid in Morrison's pouch on a trauma patient's FAST scan strongly suggests peritoneal hemorrhage, as this space is the most dependent area in the right upper quadrant where blood accumulates in supine patients. In trauma, rapid identification of blood in Morrison's pouch is critical because it indicates intra-abdominal bleeding that may require emergent surgical intervention. This finding guides timely decisions for laparotomy or further diagnostic evaluation to control hemorrhage and prevent shock. Other options like ascites, pancreatic pseudocyst, or renal cyst are less likely in acute trauma and usually present differently on imaging and clinical context.

Reference: Trauma, Keith M. Skandalakis et al., 6th Edition.