The presentation of severe epigastric pain radiating to the back, nausea, vomiting, and Grey Turner's sign (flank/ecchymosis indicating retroperitoneal bleeding) is characteristic of acute pancreatitis. Grey Turner's sign suggests hemorrhagic pancreatitis, a severe form with retroperitoneal hemorrhage. The best initial investigation is serum amylase and lipase, with lipase being more specific and sensitive for acute pancreatitis. Early diagnosis guides prompt management and helps assess severity. Other options do not typically present with Grey Turner's sign or this pain pattern.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.