A neonate presents with bilious vomiting. What is the first investigation __________
**Core Concept**
The presentation of bilious vomiting in a neonate warrants a prompt investigation to identify the underlying cause, which could be an intestinal obstruction or other gastrointestinal anomalies. The initial evaluation involves assessing the anatomical and functional integrity of the gastrointestinal tract, particularly focusing on the presence of any obstruction or malrotation.
**Why the Correct Answer is Right**
The first step in the diagnostic workup is to assess the abdominal plain X-ray (AXR) to evaluate the position of the intestinal gas, fluid, and any potential air-fluid levels. This radiographic examination helps to identify the presence of intestinal obstruction, such as duodenal atresia or pyloric stenosis, and also assesses the position of the liver edge, which can indicate liver pathology or malrotation. The AXR is a rapid, non-invasive, and readily available diagnostic tool that provides valuable information to guide further investigations.
**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is useful in evaluating the gallbladder and biliary system but is not the first-line investigation for bilious vomiting in neonates.
**Option B:** Endoscopy is not typically used as an initial investigation for neonatal bilious vomiting due to the risk of aspiration and the need for sedation.
**Option C:** CT scan is not indicated as an initial investigation due to radiation exposure concerns and the availability of safer and more informative imaging modalities.
**Clinical Pearl / High-Yield Fact**
In neonates presenting with bilious vomiting, a normal abdominal X-ray does not rule out intestinal obstruction, and further investigations, such as ultrasound or contrast studies, may be necessary to confirm the diagnosis.
**Correct Answer: A. Abdominal plain X-ray.**