**Core Concept**
Annular pancreas is a rare congenital anomaly where pancreatic tissue surrounds the second portion of the duodenum, potentially leading to duodenal obstruction. This condition can be diagnosed using various imaging modalities and surgical interventions.

**Why the Correct Answer is Right**
The correct answer states that upper GI series is not the investigation of choice (IOC) for annular pancreas. This is because upper GI series may not accurately diagnose annular pancreas due to the overlapping radiographic appearance of the pancreatic tissue and the duodenum. Instead, more specific imaging modalities such as CT, MRI, or ERCP are preferred for diagnosis. These modalities provide better visualization of the pancreatic tissue and its relationship with the duodenum.

**Why Each Wrong Option is Incorrect**
**Option B:** Duodenal obstruction is indeed present in annular pancreas, as the pancreatic tissue can compress the duodenum. Therefore, this statement is true and not the correct answer.
**Option C:** ERCP (Endoscopic Retrograde Cholangiopancreatography) is a useful diagnostic tool for annular pancreas, as it allows for direct visualization of the pancreatic duct and its relationship with the duodenum. This statement is also true and not the correct answer.
**Option D:** Non-rotation of the gut is indeed associated with annular pancreas, as this condition often occurs in conjunction with other gastrointestinal anomalies. Therefore, this statement is true and not the correct answer.

**Clinical Pearl / High-Yield Fact**
Annular pancreas is often associated with other gastrointestinal anomalies, such as duodenal atresia or stenosis, and may require surgical intervention to relieve duodenal obstruction.

**✓ Correct Answer: A. Upper GI series is IOC**