**Core Concept**
Annular pancreas is a rare congenital anomaly characterized by the presence of pancreatic tissue encircling the second part of the duodenum, which can lead to duodenal obstruction. This condition is often asymptomatic in infancy but may present with symptoms such as bilious vomiting, abdominal distension, and failure to thrive in later childhood.

**Why the Correct Answer is Right**
Bilious vomiting is not a typical presentation of annular pancreas. The obstruction caused by the pancreatic tissue is usually non-bilious, as the bile ducts are not affected by the annular pancreas. In contrast, the correct presentations of annular pancreas include non-bilious vomiting, abdominal distension, and failure to thrive.

**Why Each Wrong Option is Incorrect**
**Option A:** Annular pancreas is associated with other congenital anomalies, such as Down syndrome, Turner syndrome, and congenital heart disease. However, Down syndrome is not a specific risk factor for annular pancreas.

**Option C:** Duodeno-duodenostomy is a surgical procedure that can be used to treat annular pancreas by creating a bypass around the pancreatic tissue. This is one of the treatment options.

**Option D:** ERCP (Endoscopic Retrograde Cholangiopancreatography) is a diagnostic procedure that can be used to visualize the pancreatic and bile ducts. While ERCP can be used to diagnose annular pancreas, it is not the only diagnostic method.

**Clinical Pearl / High-Yield Fact**
Annular pancreas is often asymptomatic in infancy but may present with non-bilious vomiting, abdominal distension, and failure to thrive in later childhood. It is essential to consider annular pancreas in the differential diagnosis of duodenal obstruction, especially in patients with a history of congenital anomalies.

**✓ Correct Answer: B. Bilious vomiting**