<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A newborn is brought to the emergency department by his parents with violent vomiting. It started about 3 days ago and has slowly gotten worse. He vomits after most feedings but seems to keep some formula down. His mother notes that he is eager to feed between episodes and seems to be putting on weight. Other than an uncomplicated course of chlamydia conjunctivitis, the infant has been healthy. He was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. The physical exam is significant for a palpable mass in the right upper quadrant. What is the first-line confirmatory diagnostic test and associated finding?? A: Abdominal X-ray; double bubble sign, B: Barium upper GI series; GE junction and portion of the stomach in thorax, C: Barium upper GI series; bird beak sign and corkscrewing, D: Air enema; filling defect and coil spring sign, E: Abdominal ultrasound; elongated pyloric channel and muscle hypertrophy,  <response>: This is a question about pyloric stenosis in an infant. The key details in the stem are the infants age, history of projectile vomiting after feeding but still gaining weight, and palpable abdominal mass. These point to pyloric stenosis as the likely diagnosis. The confirmatory diagnostic test for pyloric stenosis is abdominal ultrasound, which would show an elongated pyloric channel and muscle hypertrophy. Choice A describes findings of a double bubble sign on abdominal x-ray, which is classic for duodenal atresia, not pyloric stenosis. Choice B describes findings of a barium upper GI series suggestive of a hiatal hernia, which does not fit the clinical scenario. Choice C describes corkscrew esophagus on barium swallow, which is seen in esophageal atresia. Choice D describes findings on an air enema suggestive of intussusception, which does not match the clinical picture.Therefore, choice E describing ultrasound findings of pyloric stenosis is the correct answer.E: Abdominal ultrasound; elongated pyloric channel and muscle hypertrophy 