A man post-cholecystectomy presented with jaundice, fever and dark urine. What is the most diagnostic investigation?

The key is A. ERCP [Post operative US will not give good results. We shall not go for ercp first as it has complications like injury and pancreatitis. Acceptable options are CT, MRI and MRCP among which most appropriate is MRCP! But as the Question wants most diagnostic it is ERCP. The diagnosis here is choledocolithiasis with cholangitis].