A mother is alarmed because her 6-week-old boy has begun vomiting almost immediately after every feeding. In the past week, the vomiting has grown more forceful with the vomit projecting several feet from his mouth. He is always hungry again just after vomiting. At the physician’s office, the nurse holds the child and gives him a bottle of water. While he drinks, she notes an olive-size lump in his right abdomen. Which condition should the nurse suspect in this child?