Practice question

Parents bring their 6-week-old male infant to the clinic. They report that for the past week, he has been vomiting forcefully immediately after most breastfeeds. The vomit travels a considerable distance ('projectile') and does not appear bile-stained. Despite this, he seems very hungry and feeds eagerly. On examination, you palpate a small, firm, olive-shaped mass in the epigastrium, just right of the midline. What is the most likely diagnosis?