A 3-week-old male with vomiting after feeding should undergo which investigation?

In a 3-week-old male presenting with vomiting after feeding, the most appropriate investigation is an abdominal ultrasound. This modality is particularly useful in infants as it is non-invasive, does not involve ionizing radiation, and is excellent for evaluating various abdominal conditions. In this age group, one of the main considerations for vomiting is the possibility of pyloric stenosis, a condition where the pylorus becomes narrowed, leading to gastric outlet obstruction.

An abdominal ultrasound is highly effective in diagnosing pyloric stenosis, as it allows visualization of the pyloric channel and can confirm the presence of the classic "string sign," which is indicative of the condition. It can also help rule out other possible causes of vomiting, such as malrotation or intussusception.

In contrast, while a CT scan could provide detailed images of the abdomen, it is generally not used in infants due to the exposure to radiation and the risks associated with sedation. Endoscopy is more invasive and not typically employed in the initial evaluation of vomiting in neonates. An X-ray, while useful in certain scenarios such as detecting bowel obstructions or free air from perforations, is less specific for the conditions typically explored in vomiting cases and again involves radiation exposure. Therefore, the abdominal ultrasound stands out