What is the most likely location of obstruction in a 5-week-old infant presenting with projectile vomiting after feedings, weight loss, and decreased urine output, with a history of recent introduction to cow milk-based formula?

Differential Diagnosis for a 5-week-old Boy with Vomiting after Feedings

• Single most likely diagnosis: + Pyloric junction (E): The patient's symptoms of vomiting immediately after feedings, with a history of minimal spitting up until recently, and significant weight loss, are classic for pyloric stenosis. This condition typically presents at 3-6 weeks of age and is characterized by an obstruction at the pyloric junction due to hypertrophy of the pyloric muscle.
• Other Likely diagnoses: + Duodenum (A): Duodenal atresia or stenosis could cause similar symptoms, but it usually presents earlier in life, often with bilious vomiting. + Gastroesophageal junction (B): Gastroesophageal reflux disease (GERD) could cause vomiting, but it typically does not result in significant weight loss or dehydration as seen in this patient.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.): + Jejunum (C): Intestinal obstruction due to conditions like jejunal atresia or volvulus could present with vomiting and dehydration. Although less likely, missing this diagnosis could be catastrophic. + Midesophagus (D): Esophageal atresia or stenosis could cause vomiting, but it usually presents immediately after birth or with dysphagia, and is less likely given the patient's age and history.
• Rare diagnoses: + Other rare conditions like annular pancreas, malrotation, or congenital gastric volvulus could also cause intestinal obstruction and vomiting, but these are less common and typically present with additional symptoms or signs.