This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: GIT and hepatobiliary system, Subject name: Radiology
[{'content': 'Double track sign is seen in - \nSelect and explain. \nA. Duodenal atresia, B. CHPS, C. Gastric ulcer, D. Achalasia', 'role': 'user'}, {'content': 'Answer: B \nExplanation: On upper gastrointestinal fluoroscopy: * delayed gastric emptying * peristaltic waves (caterpillar sign) * elongated pylorus with a narrow lumen (string sign) which may appear duplicated due to puckering of the mucosa (double-track sign) * the pylorus indents the contrast-filled antrum (shoulder sign) or base of the duodenal bulb (mushroom sign) the entrance to the pylorus may be beak-shaped (beak sign)', 'role': 'assistant'}]