What condition is characterized by bilious vomiting and peritoneal signs in a week-old baby?

The condition characterized by bilious vomiting and peritoneal signs in a week-old baby is intestinal malrotation. This condition arises from the abnormal positioning of the intestines during fetal development, which can lead to complications such as volvulus, where the intestines twist around themselves. This twisting can cause obstruction, leading to symptoms such as bilious vomiting, which indicates that bile is present in the vomit due to blockage or backflow.

Peritoneal signs, which may include findings like abdominal tenderness or rigidity, suggest that there is irritation or inflammation in the peritoneal cavity, often a result of ischemia caused by compromised blood flow due to the malrotation. Prompt diagnosis and surgical intervention are critical in such cases to prevent severe complications, including bowel necrosis.

While meconium ileus is associated with cystic fibrosis and typically presents with a different clinical picture, pyloric stenosis primarily causes projectile non-bilious vomiting and duodenal atresia often presents with "double bubble" sign on imaging along with bilious vomiting. However, the presence of peritoneal signs strongly indicates intestinal malrotation as the underlying issue here.