**Core Concept**
The question is testing the knowledge of non-invasive diagnostic tests to assess the patency of a surgically created bile duct bypass, specifically a choledochoduodenostomy. The patient's presentation of RUQ abdominal pain suggests possible bile duct obstruction.

**Why the Correct Answer is Right**
A HIDA (Hepatobiliary Iminodiacetic Acid) scan is a nuclear medicine test that evaluates the function and patency of the biliary system. It involves injecting a radiolabeled compound that is taken up by the liver and excreted into the bile. A functional choledochoduodenostomy would allow the radiotracer to flow from the liver, through the bile ducts, and into the duodenum, demonstrating patency of the bypass. This test is non-invasive, has a high yield in diagnosing biliary obstruction, and can be performed quickly.

**Why Each Wrong Option is Incorrect**
**Option A:** ERCP (Endoscopic Retrograde Cholangiopancreatography) is an invasive test that involves inserting an endoscope into the stomach and duodenum to visualize the bile and pancreatic ducts. While it can diagnose and treat biliary obstruction, it is more invasive than a HIDA scan and carries a higher risk of complications.

**Option B:** Percutaneous transhepatic cholangiogram (PTC) involves injecting contrast material directly into the bile ducts through the skin and liver. It is an invasive test that carries risks of bleeding, infection, and bile duct injury. PTC is not the initial test of choice for evaluating patency of a choledochoduodenostomy.

**Option D:** A CT scan of the abdomen can provide information on the anatomy and possible obstruction of the bile ducts but is not the best test for evaluating the functional patency of a choledochoduodenostomy.

**Clinical Pearl / High-Yield Fact**
A HIDA scan is a useful test for evaluating biliary obstruction, especially in patients with a history of abdominal surgery, as it is non-invasive and can provide functional information on the biliary system.

**✓ Correct Answer: C. HIDA scan**