This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pancreas, Subject name: Surgery
A 70-year-old male underwent a choledochoduodenostomy for multiple common duct stones. The patient now presents with RUQ abdominal pain. What should be the initial test (least invasive with the best yield) to determine patency of the choledochoduodenostomy? 
Select and explain. 
A. ERCP, B. Percutaneous transhepatic cholangiogram (PTC), C. HIDA scan, D. CT scan of the abdomen
C: HIDA scan
Answer: C: HIDA scan 
Explanation: A HIDA scan will show excretion of the radiolabeled isotope into the biliary tree, but there will be no flow into the duodenum, indicating that the biliary-enteric anastomosis is occluded. If an upper GI study with barium is performed, visualization of the common bile duct would indicate patency of the choledochoduodenal anastomosis.