This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Intestinal obstruction, Subject name: Surgery
A women of 35-years, comes to emergency depament with symptoms of pain in abdomen and bilious vomiting but no distension of bowel. Abdominal X-ray showed no air fluid levels. Diagnosis is 
Select and explain. 
A. CA rectum, B. Duodenal obstruction, C. Adynamic ileus, D. Pseudo-obstruction
B: Duodenal obstruction
Answer: B: Duodenal obstruction 
Explanation: Abdominal pain, bilious vomiting without abdominal distention is suggestive of proximal small intestinal obstruction, distal to ampulla of Vater (Duodenal obstruction) Nausea and vomiting are more common with proximal obstruction Abdominal distention is more common in distal obstruction