A 41-year-old woman has 18 hours of nausea, right upper quadrant abdominal pain, and fever. She ate a heavy meal the night before and has never experienced similar symptoms. Laboratory evaluation reveals an elevated white # blood cell count, normal bilirubin, and sightly elevated aspartate aminotransferase (AST) and alanine aminotransferase (ALT). What is the most appropriate initial imaging study to determine the etiology of her symptoms?