A 4-week-old female infant vomits after feeds and has a palpable mass. What is the most appropriate next step?

In the case of a 4-week-old female infant who vomits after feeds and has a palpable mass, the symptoms are suggestive of hypertrophic pyloric stenosis, a condition commonly seen in infants that leads to projectile vomiting and a "olive" shaped mass in the abdomen due to hypertrophy of the pyloric muscle.

Determining the appropriate next step in management is crucial. The most appropriate next step would typically be to conduct an abdominal ultrasound, as it is a non-invasive method that can effectively identify pyloric stenosis by showing a thickened pylorus. The other options provided do not address the underlying issue as effectively.

For example, checking serum potassium levels can be important in assessing electrolyte imbalances often resulting from persistent vomiting, but it does not directly evaluate the anatomical issue causing the vomiting. Likewise, nebulised salbutamol is used primarily for respiratory conditions and is not relevant in this context. Lastly, while intravenous fluids might be warranted to correct any dehydration that could arise from vomiting, addressing the diagnosis itself should take precedence, making abdominal ultrasound the appropriate next step.

Identifying the underlying cause of the symptoms is critical, especially in a vital and developing infant, hence the abdominal ultrasound is necessary.