Caterpillar Sign in an Infant with Hypertrophic Pyloric Stenosis

A 7-week-old boy presented to the emergency department with a 2-week history of emesis. He was born at 39 weeks of gestation with a birth weight of 3495 g via normal spontaneous vaginal delivery. He had no history of taking macrolide antibiotics. He presented with postprandial, projectile, nonbilious vomitus after vigorously feeding on breast or formula milk. His weight decreased by 220 g, suggesting more than 5% dehydration over the previous 2 weeks. Physical examination revealed dry mucous membranes, but no abdominal mass, abdominal distension, or high-pitched bowel sounds.

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